Are there any recommended guidelines if trying to open a project created in 5.6 in 5.5?
Should you delete the entire Library folder first before trying to bake new light maps?
Projects made with a newer version of Unity won’t open in older versions of Unity.
This has always been the case and probably won’t change in the foreseeable future.
Not true. I just opened a 5.6.1 project with 5.5.3. It automatically imported some assets and works fine.
Though I believe it’s not supported and will give undefined results. It may work ok on some project but may be totally broken on others - it really depends if you’ve used a feature/component/etc that will break when going backwards.
OP, I’d certainly delete the Library folder before trying this, I even suggest doing so when going up versions - it’s all regenerated anyway so will provide the most reliable results.