Actually, you are better off on 1.47.0000 S-ON than 1.49.0000.
The best way, frankly, is to install the root ROM PBM00IMG.zip. This will reinstall stock recovery and completely wipe data, and will also install an old radio (so you should then run something to update that as well.)
Anyway, steps would be:
- install root ROM ROM PB00IMG.zip
- reroot the phone
- install Flashback_21v2 with radio to go back to stock, but update the radio as well
- reroot again
- Nandroid restore what you have now, install whichever ROM you wish, etc.
A link to the root ROM PB00IMG.zip is here:
[HTC ROMs] Consolidated HTC Eris RUU/OTA/ROM URLs - xda-developers - remember to rename it to PB00IMG.zip on the SD card (that's zero-zero, not two letter o). Restart the phone in HBOOT (hold VolDn while starting the phone) and it should walk you through installing the image. A photo walkthrough of this process is here:
Visual Example: The PB00IMG.ZIP update method - Android Forums
Also, remember to rename or delete the PB00IMG.zip file after you restart, so it doesn't accidentally run again.
To get Recovery back on, you can get it here, with instructions:
Amon_RA 1.6.2 Custom Recovery: trackball not required! - Android Forums - look toward the bottom of that post - ScaryAlien has a link to a version of jcase's 1-click app. Or, you can try fastboot if you have the SDK installed.
Flashback_21v2 is here:
[ROM][03/06/2011]FlashBack21 v2 -Return to Factory Stock - March 2011 OTA - xda-developers - read the part after the red headline about flashing the radio. Make sure you get the radio version, not the noradio version - that's the only reason you do this step.
And, of course, you can reroot with the same method as above.