replacing 121 rails and adding 144 extensions or just..

[Vadimb]

09 Nytro RTX. Rails are worn through the hyfax and the aluminum pretty bad.
So I decided to replace them and extend to 144" track. Should I just buy XTX rails instead? the cost would be comparable.
Will the 144 rails fit 121 suspension without modifications?
Any other problems with it?
Thank you