Can you make a 25k a 50k? I know you can do the opposite but I have a 12mm 25k pot I need to make into a 50k
YOu can make it 50K total, with only a 25K range between 25K and 50K but otherwise, no, you cannot make a pot have a range larger than its manufactured value.
Depending on the specific use of the pot in the circuit in question, you MIGHT get away with a substitution. Might.