I prefer fugumod. Its one of the most rapid development for GB kernal currently.. If u are on k15a, i think you can upgrade it directly to fugu kernal. Try to wipe dalvik cache and wipe cache partition just to be safe after flashing the kernal.

Fugumod is one of the slickest kernal for GB ive tried so far (and also its the only kernal supports voodoo sound 7!)

U can always back up your apn using titanium backup

Hi. Corrections here. If u flash fugu kernal, it wont straight away convert to jfs. Its still in rfs. Maybe u confused because when u open fuguapp it appears only for ext4/jfs. The best way to know whether u are in jfs/ext4/rfs or whatever filesystem is by checking through sdcard>fuguapp>current.fs. Open with root explorer.
To change from fugu to other kernal, u might as well change back to rfs (besides ext4). But u need to modify the current.option first.

From what i know, fugumod its not compatible with other kernal. If u try to revert/convert to another kernal from fugumod, it might cause some problems (just like the one u have right now.) If i were u, i would start back from fresh..