I plan to transfer my domain names from one register host to another. What is the procedure in undertaking the transfer? Is the transfer expected to affect my sites' uptime?
I'm not sure what you mean but these are the possible answers: If you are only transferring the domain name, it should not affect your website performance, however if your hosting service is the registrar for your domain, it's up to such service let you transfer away or deny it. You would have to tell us what your hosting is to tell you how to make the transfer. Otherwise, if what you are planning is move your domain name from one hosting service to another hosting, you may ask your new hosting server to attempt to move your site to avoid downtime, otherwise upload your files to the new hosting and don't update nameservers until you have everything in place so if there is any downtime, it won't be a big deal.
If you change your web host you will need to change your name servers only. No need to change something else.
Well, uptime is concerned to your hosting package and changing the registrar is a whole differing thing.