preserving grub/suse 9.3 --> upgrading to slackware 10.2

Hello friends! Today I am using Grub from SuSE 9.3 to boot my machine — it's my choice; I like the graphics from this distro. Inside my PC there are SuSE 9.3, Slackware 10.1 and Windows 98SE (no comments please). I just want to upgrade to Slackware 10.2, but I'm afraid it can undo the way my machine is starting now. How can I upgrade Slackware and preserving Grub also? Thanks for any help Adagilson