I need to have my 3GS replaced unbder warranty but want to know what os i'm likely to get on the replacement and if i will be able to jailbreak it as i have my existing one?
you can jailbreak for sure since all jailbreaks are out. can't tell you the unlock situation or what firmware you will get that's a guess. why bother you might get lucky and the other owner jailbroke it and saved shsh blobs. really nobody knows. after you get it confer here.
I had my iPhone 3GS replaced that only had 4.2.1 SHSH blobs. The replacement had 4.2.1 and 4.3b1 SHSH blobs... Then I had to get that phone replaced, and this current iPhone 3GS has 4.1 and 4.2.1 SHSH blobs! I can go back down to 4.1 if I have to!
no there is no way nor reason to delete them. even if you restore your device and don't jailbreak again it's still good to have the blobs. it's the only way you can restore older firmware if needed on a 3GS or I4 if nothing else it adds value or desire to your phone on resale. 😉