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I'm considering putting a custom ROM on my SGS. Are there any ROMs that enable me to install updates without wiping the entire phone?

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Have you had specific problems with this? Many ROMs will work fine when updating from one version to another. For example, the instructions on CyanogenMod's forums (taking the EVO as an example) say:

Upgrading from earlier version of CyanogenMod 7?

  1. Perform a NANDroid backup of your current ROM.
  2. Flash CyanogenMod (your Google Apps will be backed up & restored automatically).

Specifically they're referring to a case such as upgrading from say 7.0 to 7.1. The NAND backup is just in the event that you want to roll back or something goes wrong, but in reality you could even consider that optional (though highly encouraged).

Again taking CM as an example, you usually only need to wipe your phone if you're going from one major version to another. A good example would be if you're going from an Android 2.2-based ROM to an Android-2.3 based ROM. This is because there are some significant changes under the hood to Android, and is not really related to the specific ROM itself in most cases. The exception to this is typically carrier OTA updates since they specifically tailor them to play nice with the previous versions (end users wouldn't want to wipe their phone for every OTA, obviously).

Edit: Just to clarify, what I mean by the above is that CM won't wipe your /data partition (or /cache, I believe) on its own, as described here. It will also backup Google Apps and reinstall them to /system during the update. You can wipe /data manually from your recovery or ROM Manager, but you certainly don't have to. I have read, though, that using ROM Manager to flash an update will automatically wipe /data. There may be a setting for this or this may not be exactly true - I am not sure because I do not install my ROMs this way (I boot into recovery and do it manually).

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I haven't tried it yet. I want to know if there are versions I should avoid. –  Nathan Fellman Jul 11 '11 at 13:56
    
@Nathan: I can't speak for most ROMs, then, since I've been using CM for a long time, but as noted above, CM does fall into this category. The only exception is moving between major versions (like 6.x to 7.x) since they change the base Android version at that point. I suppose you could always try making a backup and updating a ROM you're interested in without wiping. If it works, great! If not, restore and try another. –  eldarerathis Jul 11 '11 at 14:00
    
I use a custom ROM called Bionix and new versions, even minor updates, wipe most everything. CM sounds like it wipes too, it just has built-in restore of some apps. I'm not sure this is exactly what Nathan's looking for, it sounds like he's looking more for an OTA-like update that normally does no damage. –  Matthew Read Jul 11 '11 at 15:15
    
@Matthew: CM doesn't wipe /data (or /cache to my knowledge) when you install it, only /system, but it restores Google apps on /system if you already have them. It's pretty much like an OTA in that regard (settings, contacts, Market apps, etc are retained). I'm not sure exactly what OTAs to do /system, though, so I don't know how different that is. –  eldarerathis Jul 11 '11 at 15:31
    
Ah OK, that's good :P –  Matthew Read Jul 11 '11 at 16:01
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