Just to add on to this -- when you have a model number, you can still find Shimano technical documents through their own website. You can use their exploded diagrams to validate whether a Uniglide hub you're considering uses a fixing bolt to hold the freehub body or just uses the locknut assembly to hold together. If it uses the fixing bolt, the body can be replaced with a Hyperglide body. There may be fit issues with the seal between a specific freehub body and a specific hub shell -- I'm not knowledgeable enough about those fitment issues to be helpful, but it has been covered in other threads, so there is information available.