An elephant sat on my T-Mobile GSM phone, and I need to replace it.
T-Mo will let me upgrade to a Nokia 6101 for only $170 (cough, cough) if
 I sign a 1-year contract extension.

I've seen the same phone for sale around town and on Amazon for use with
T-Mobile's prepaid "To Go" product, for $145-$150.  If I buy a T-Mo
prepaid Nokia 6101, can I put my own (contract) T-Mo SIM in it?  Or are
the prepaid Nokia's modified in some way at the factory to prevent such
use (or perhaps are they recognizable as prepaid-only to the T-Mo
network, regardless of the SIM inside)?

Thanks for any tips!