1. well first you have to make sure your ps3 is qa flagged by doing this:
Set your cursor on (not in) Network Settings and press the key combo (all at the same time): L1+L2+L3 (press_left_stick) +R1+R2+dpad_down
If its not flagged then your going to need to flag it by following the QA dehashing method (this will make it a lot safer to update the ps3 using cfw)
After you dehash it then you want to get the cfw you want on your ps3 downloaded and ready to install. then you want to get the newest multiman app for the ps3 and move it to the root of the usb stick and install it then the new cfw that you would like over the fw that you installed on 3.55 (after dehashing the system or else you might get a brick) then you will be ready to play the newest games without (much) hassle
2. the downgraders are used for upgrading/changing the cfw on the ps3 (to make it a lot safer) and not have the chance to brick it
Just a question .... Why would you want to upgrade your CFW just to play Heavy Rain? .. I have 3.55 Kmeaw CFW installed and have already played Heavy Rain ... unless there is an update or DLC that requires something different...
One more thing ... this post is somewhat useful if you need to know all the steps involved for a 3.55 CFW...