You can always re-install whatever firmware you're on.. You're on 3.55 so it doesn't matter what variation (Kmeaw, Rebug, JB2 etc) you're using or want to switch to.
You're safest bet is to re-install OFW 3.55 first, then install the other FW you want. So, you're on Kmeaw now, install OFW 3.55, then install JB2. To return to Kmeaw, install 3.55 OFW, then install Kmeaw.
You're safest bet is to install these from recovery mode, Google how to get to that menu if you don't know how.
It gets more complicated when you want to run say, a 3.41 CFW or clean the PS3 before updating to 3.73.
Also, don't forget to take loads of pictures and videos when it arrives! =P
: You need to get a Progskeet
and use that to downgrade, if you're not confident at soldering, check out 360-clip.com for a NAND clip (they'll be releasing a NOR clip soon). To find out which one you need, if it's a Phat there's a sticker at the back and it'll say CECHX
01 and the X is what's important. CECHA-CECHH are NAND, everything else is NOR (including all slims).