If you want to update but aren't 100% you'll stay, make a backup of 4.8 before you do (more importantly, back up those installers that you got with DIM or however you installed them...including the plugins) Once you install 4.9, unless you manually ensure otherwise, your old install is overwritten.

Do be aware, some characters sold at DAZ look like crap in 4.9 because of the big change in IRAY. I have 3 or 4 G3F characters that look awful. I've tried the fixes that they mention in the forums (various changes to the shaders), but it doesn't help. So, I am still using 4.8 and will do so until the ones I use are updated. I'm finding connect doesn't work with my two computer setup (1 for creating scenes, and 1 for rendering them) so I am mostly happy with 4.8, except the rare occasion where IRAY crashes DS. I don't plan to buy any connect only content until DAZ makes it easier to share connect content over a local network (right now, you have to install the content on each machine, which is a hassle I don't want to mess with).