3GS old bootrom, old BB: 4.0 JB + unlocked --> 4.2.1 JB (+ unlock if possible)

[DylPickle85]

Hi guys,
I wanted to run my scenario by you and see if you had advice if I am on the right track to successfully JB'ing my iPhone with minimal issues.

Here's my current scenario:
3GS, old bootrom, 04.26.08 baseband, 4.0 JB with sn0wbreeze, unlocked with ultrasn0w.

I want to update to 4.2.1 + JB + unlock if possible.
It seems like redsn0w or GP are my two options, but I would like to maintain my baseband and keep GPS if possible. Keeping unlock capability is less of a desire at this point.

My tentative approach is as follows:
0) Backup phone in iTunes + PKGbackup for JB apps
1) Perform standard FW update to 4.2.1 via iTunes
2) JB with greenpois0n RC5_b4

will this approach preserve my BB, and give untethered JB, but no unlock capability?
Is it worth keeping current BB, or upgrading to iPad BB 6.15? I'd hate to lose GPS!

 

[WhatAmI]

You should never cross post. Your question has already been answered.

 

[DylPickle85]

was going to delete from the other thread before you replied... will delete now

edit: buhhh -- cant figure this out, mods please delete this thread if possible