Currently, Metamorph (and its survival-based fork Changeling) are both available for Minecraft versions 1.12.2 and older. Minecraft version 1.12.2 was released over 4 and a half years ago as of the time of writing, with there being six stable major version releases since then.

The current version targets for Forge are 1.18 (latest release with mainstream support) and 1.16 (LTS).

Would it be possible to port Metamorph and/or Changeling to the more modern versions of Minecraft? Do note there's been a lot of changes in the source code for both Minecraft and Forge since 1.12 which the mod will need to be adapted for.
For example, modern versions of Minecraft are a lot more data driven.

Changeling is not by McHorse. We cannot update forks of this mod. The changeling owner said he might gradually open pull requests to this mod so the features will be integrated into upstream.

Secondly, porting metamorph would require a huge amount of time which I do not have. Also, when porting metamorph, people would want the other mods to be ported, which would in total probably be 300.000 lines or more of code to update. I will not do this alone and not now.

Currently, Metamorph (and its survival-based fork Changeling) are both available for Minecraft versions 1.12.2 and older. Minecraft version 1.12.2 was released over 4 and a half years ago as of the time of writing, with there being six stable major version releases since then.

The current version targets for Forge are 1.18 (latest release with mainstream support) and 1.16 (LTS).

Would it be possible to port Metamorph and/or Changeling to the more modern versions of Minecraft? Do note there's been a lot of changes in the source code for both Minecraft and Forge since 1.12 which the mod will need to be adapted for. For example, modern versions of Minecraft are a lot more data driven.

As @Chryfi mentioned, it’s a colossal amount of work that isn’t worth it. He’s currently the main maintainer of my mods, while I completely quit Minecraft modding, so there is a slim chance that someone will take and port the mod for free (or even for considerable fee).