In theory you can migrate between BGSes, but you can always just use one at any point in time.
In practice no one will switch because it makes no sense to do it. If there happen to ever be more than one real BGS contender, it will be from something like Cloudflare that will just replicate everything Bluesky Inc decides.
I don't know if it does not make sense. AFAIU these BGSes could be special-purposed e.g. for a business, community or topic of interest. Why wouldn't it make sense to synchronise the collected data between these BGSes and get a combined view on the data? With just a single BGS we have another centralized big tech platform. I think decentralized BGSes are a major factor in how interested people are in becoming part of the ecosystem.
In practice no one will switch because it makes no sense to do it. If there happen to ever be more than one real BGS contender, it will be from something like Cloudflare that will just replicate everything Bluesky Inc decides.