(JourneyMap instructions are WIP. Please check back later.)
When you switch minimap mods, transferring your waypoints you made can be troublesome. Don't worry, this guide will make it easier for you!

The waypoint data

In order to transfer your waypoints efficiently, you need to understand how waypoint data are stored.

VoxelMap's waypoint data is stored in %appdata%/.minecraft/mods/mamiyaotaru/voxelmap/[​world name].points. Each line has one point.
name:name,x:x,z:z,y:y,enabled:boolean,red:r,green:g,blue:b,suffix:suffix,world:world,dimensions:dimension#Xaero Minimap's waypoint data is stored in %appdata%/.minecraft/XaeroWaypoints/[​world name]/[​dimension]/waypoints.txt. Each line has one waypoint.
waypoint:name:initials:x:y:z:color:disabled:type:set:rotate_on_tp:tp_yaw:globalJourneyMap's waypoint data is stored in %appdata%/.minecraft/journeymap/data/sp/[​world name]/waypoints. There is a JSON file for each waypoint. It is usually named "[​name]-[​x]-[​y]-[​z].json". In each file, it contains the following data:
{
"id": [id, usually "(name)-(x)-(y)-(z)"],
"name": [name],
"icon": [path to said icon],
"colorizedIcon": [path],
"x": [x],
"y": [y],
"z": [z],
"r": [r, 0 to 255],
"g": [g, 0 to 255],
"b": [b, 0 to 255],
"enable": [true or false],
"type": [type],
"origin": [origin],
"dimensions": [
[applicable dimensions]
],
"persistent": [true or false],
"showDeviation": [true or false],
"iconColor": -1,
"customIconColor": [true or false]
}
Thank you to xaero96 for providing the rest of Xaero Minimap's data for me.

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Converting waypoint data to Xaero's Minimap

VoxelMap to Xaero's Minimap

Navigate to %appdata%/.minecraft/mods/mamiyaotaru/voxelmap. You will see many .points files for each world. Look for the world you want, and open said world's waypoints file with text editing software. You will see lots of data.

Then, navigate to %appdata%/.minecraft/XaeroWaypoints/. In a new window. If you don't see a folder with said world name, create a new folder. As you see, the VoxelMap's points file puts all the waypoints in a single file, with a parameter for the dimensions. Create a new folder for each dimension. Then, in each dimension, create a text file called "waypoints".

The red, green, blue, and suffix parameters are not needed. I would recommend approximating VoxelMap's waypoint colors to XaeroMap's waypoint colors, which is based off Minecraft's color codes. Make sure you input it in decimal, not hexadecimal! Usually, the initial is the first letter of its name, but it can be any initial.
For the enabled parameter, in order to convert it to Xaero's Minimap disabled parameter, you have to invert it. If it's true in VoxelMap, then it's false in Xaero's Minimap and vice versa.

If your waypoint data is:
name:Reviloja,x:-753,z:753,y:76,enabled:true,red:0.4117647,green:0.22745098,blue:0.0,suffix:,world:,dimensions:overworld# then Xaero's Minimap waypoint data is:
waypoint:Reviloja:R:-753:76:-753:4:false:0:gui.xaero_default:false:0
For the world paramater, I haven't figured out what it does. If you do, please tell me in the comments.
For the dimensions paramater, simply put the waypoint data on the appropriate dimension. If said dimension does not exist, create a new folder with the dimension and make a text file named "waypoints". If there are multiple dimensions indicated in VoxelMap's waypoint data, copy the waypoint data to the appropriate dimension folders in Xaero's Minimap folder.

Now, you will repeat for each waypoint.
By default, they are shown in gui.xaero_default set.
If Minecraft is running, please quit the game and restart the game.

If you have followed the instructions properly, this should show:

JourneyMap to Xaero's Minimap

Navigate to %appdata%/.minecraft/journeymap/data/sp/[​world name]/waypoints. You can see a JSON file for each waypoint in the world.

Then, navigate to %appdata%/.minecraft/XaeroWaypoints/. In a new window. If you don't see a folder with said world name, create a new folder.

The id, icon, colorizedIcon, type, origin, persistent, showDeviation, iconColor and customIconColor aren't important for this.
For the r, g, b, I would recommend choosing a color that is close to JourneyMap waypoint's color.
For the enable parameter, in order to convert it to Xaero's Minimap disabled parameter, you have to invert it. If it's true in JourneyMap, then it's false in Xaero's Minimap and vice versa.

If your waypoint data is:
{
"id": "Reviloja_-753-76-753",
"name": "Reviloja",
"icon": "journeymap:ui/img/waypoint-icon.png",
"colorizedIcon": "fake:color--16727810-waypoint-icon.png",
"x": -753,
"y": 76,
"z": -753,
"r": 105,
"g": 58,
"b": 0,
"enable": true,
"type": "Normal",
"origin": "journeymap",
"dimensions": [
"minecraft:overworld"
],
"persistent": true,
"showDeviation": false,
"iconColor": -1,
"customIconColor": false
}then Xaero's Minimap waypoint data is:
waypoint:Reviloja:R:-753:76:-753:4:false:0:gui.xaero_default:false:0Repeat this procedure for each waypoint you want to convert. Make sure to put all the converted waypoints in a single file, 1 per line.
If Minecraft is running, close it and then restart the game.

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Converting waypoint data to VoxelMap

Xaero's Minimap to VoxelMap

Navigate to %appdata%/.minecraft/XaeroWaypoints/[​world name]/[​dimension]/. You can see a text file called "waypoints".

Then, navigate to (username)/AppData/Roaming/.minecraft/mods/mamiyaotaru/voxelmap in a new window. If there isn't a .points file with this name, create a new .points file called "(world name).points".

For color paramater, you can closely approximate the color in Xaero's Minimap with the red, green, and blue paramaters. Make sure to use decimals in 0-1 range, where 0 is 0 and 1 is 255.

But how do you convert waypoints in different dimensions? Well, it is handled by VoxelMap's dimensions parameter. You put the applicable dimensions in said parameter depending on where you find the points in said dimension folder.

Let's assume I want to convert this:
waypoint:Redlands:R:395:94:-306:12:false:0:gui.xaero_default:false:0to VoxelMap. I will follow the instructions, and this is the result:
name:Redlands,x:395,z:-306,y:94,enabled:true,red:0.5,green:1,blue:0.35,suffix:,,dimensions:overworld#Do not forget the # at the end!
Repeat this procedure for each waypoint.
Save the file. If Minecraft is running, quit and then re-launch Minecraft.

JourneyMap to VoxelMap

(coming soon)

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Converting waypoint data to JourneyMap

(coming soon)