While it comes with some risks, it probably wouldn't be super difficult to update the views/filters (PIPELINESELECTIONS) to point to the new user IDs for each user with an appropriate query? IIRC there should be max one row per user (can't recall if it's 1-to-(0,1) or 1-to-1 from USERS).
Not aware of any specific export or share support. The UI uses an internal API /go/api/internal/pipeline_selection to retrieve (GET) and update (PUT) the views in one big block as an array of "filters", which in theory you could use to get a JSON representation of your individual views if you can still login with the old username. If one is more savvy, one could then also PUT the collection back to the same API to update views when authenticated with the new username - but obviously this is undocumented/unsupported and would require some browser "Inspect" digging :-)