Change ports to port forward?

I want to change the ports in Syncthing to be able to port forward. I use a VPN and my VPN allows me to have access to certain port numbers that are provided by the VPN.

Is it possible to change the port numbers in syncthing to a port number that is provided by my VPN to be able to port forward with a VPN?

Yes, all ports are configurable. Actions -> Settings -> Connections -> Sync Protocol Listen Addresses. See the documentation for the format and defaults:

https://docs.syncthing.net/users/config.html#listen-addresses

(This does assume that your port numbers are more or less fixed though. If they change frequently, there’s no way to update them automatically unless your script that yourself)

Would default:55555 be valid for an open port? My IP address may change often but my open port number will not.

This would be treated as listen on a hostname called “default”. If you mean “listen on all/any interfaces, port 55555”, you can use tcp://:55555

Could I do both tcp and udp on port 55555 with any IP address?

This is all explained on Syncthing Configuration — Syncthing documentation.

Syncthing only uses UDP for QUIC (which you may or may not want to use), but yes the syntax is the same.