>>394230191
For maps with little scripting it should be easy. It's already very easy to migrate a map up a UE4 version and UE5 is mostly just a continuation of UE4. There will be a lengthy UE5 beta build going up for mappers to prepare their maps in advance. Custom gamemodes might be impacted but I don't think UE5 changes blueprints so much that it will horribly break things without indicating where you're using obselete stuff. 
It's the old and abandoned workshop content that we risk losing but if anything there is of value it will be remade by someone else, if you want proof of this there are always people trying to port an already ported map.

The eventual steam workshop replacement (that we know nothing about yet and may end up signficiantly delayed) is probably going to be a complete clusterfuck.