But it has dropped off the dropdown - how can I export a 1.55 version now ?
1.56 and beyond give me shader errors I am not cable of fixing 
[EDIT] - I’ve read I can just swap out the playcanvas js file in the download zip. ok !
That’s correct, yes
If you are using the REST API, there’s also an engine version param to make this easier for you
You can also set the engine version in the launch tab if need be to via use_local_engine Using a Custom Engine | Learn PlayCanvas
We can’t guarantee backwards compatibly though. We do have a shader chunk migration guide that can help with chunk changes: Shader Chunk Migrations | Learn PlayCanvas