Have a previously jailbroken 3GS, not sure how to reset old data and keep jailbreak

i have an old 3GS that i jailbroke and unlocked (for T-Mobile) probably over a year ago. I want to give it to a family member who is also on T-Mobile so I need to reset my personal data and keep the jailbreak/unlock. I have been out of the iOS loop for a long time (switched to WM7) so i am not sure what i need to do exactly since most of the threads are about iphone4(S).. here is what i know

-16gb 3GS (MC131LL)
-Modem: 06.15.00
-iOS: 4.1 (8B117)
-Currently jailbroken and unlocked (have access to Cydia)

is it safe to just do a "Erase all content and settings"?

Since you're on 06.15, you're going to have to shift-restore a custom No_BB firmware in Pwned DFU mode anyways to get anything onto the device. Once you do that, all the data is wiped, just don't restore it from your backup. Erase all content and settings will bootloop the phone until you do what I suggest anyway, so I don't recommend it. :)