Yea they can't be re-tuned. I took apart one of my Nokia 5190's here, and it has specialized filters just for the frequency they operate at:
https://i.imgur.com/xUP9ZDG.jpg
On another note, I'm leaving MetroPCS, and switching to Mint Mobile. Entering my Nokia 5190's IMEI number into Mint's phone checker says it IS compatible and identified it properly as a 2G phone. So I guess we'll see if the Nokia gets to keep ticking. I'll find out maybe Tuesday, if the number ports fast enough. (I don't see why not, because they are using T-Mobile's network so technically I'd be hitting the same towers).