Okay so here's my latest dilemma,

I recently lost my beloved Droid Incredible to a pool, and while morning it's loss I am stuck using a Droid Eris that my sister used to use until I find a better long-term solution to replace my DINC. Well, being used to Gingerbread I didn't really feel a need to install a new ROM on my now-water-logged Incredible and thus have zero experience flashing a ROM. But the Eris, as I'm sure you know, is on Android 2.1, and with limited hardware I pretty much need to have the ability to move apps to the SD card or I will go insane for the next month or two (Please don't suggest upgrading yet as I'm absolutely not upgrading for at the very least 1-2 months away); so to live with the Eris I'm going to put a new ROM on it to at least Gingerbread. Now for my real questions.

I have already rooted it and have been enjoying root access and ClockworkMod custom recovery, but to flash a ROM do I need S-Off? S is currently on in my bootloader. I've been Googling and searching forums for a definitive answer but most don't seem to cover if I need S-Off at all to flash a ROM. And most Root and ROM guides treat it as though you simply root and then you're in ROM-land!

Next, if I do need S-Off to flash a ROM then can I get S-Off already being rooted? If I get S-Off will I need to re-root again? And most importantly, if I go to get S-Off will I need to un-root to do so?

I would consider myself an intermediate Android user and as long as the directions are clear I'm not afraid to get a little technical, but I also don't want to get in over my head if I'm trying to flash a ROM and I run into problems because what I'm doing is bricking my phone. I can't afford to lose this phone as it's my only phone for the time being; in the future I'll be more free to play with it and get more dangerous in my tinkering when I have a stable primary phone and this is just for fun.

Anyway, thanks, and any feedback is welcome. But if you don't know what the heck you're talking about then please don't tell me to do this or that, I need serious responses ASAP as I'm hoping to flash a ROM as soon as I can so I can finally breathe on a currently limited phone.