Potential to change to a '6-pack' arrangement depends on the model year. The earliest 172s, '56-'58 had the instruments spread out across the full width of the panel (like the 140s and 195s from the late 40s). Radios and switches were mounted directly below, so these will be the hardest to convert to a modern panel. (And as a personal request - please don't chop up the panels on these antiques - they deserve to be left original!)
Then in '59 Cessna revised the panel, and while the layout wasn't the standard 6-pack arrangement we know today, it WAS in a basic grid with all of the primary and secondary instruments directly in front of the pilot. Everything is mounted in a non-structural, floating panel that can easily be replaced and you can position the instruments in a conventional layout very easily.
In '64 (maybe '63?) they changed again and went to the 'center stack' radio rack which also meant changing the control column behind the panel from the older 'T' style to the modern 'U' shape. Still not the modern 6-pack, but the panel is more roomy and easily updated as with the previous generation.
Finally in '67 they implemented a modern 6-pack arrangement. That was pretty much it until the G1000 came along.
Just my personal opinion here, but I'd say anything '59 or later would be fine for learning IFR, even with a non-standard layout. All the info is there, you will learn your scan accordingly.
C.