I currently have 120mm forks but i have seen a nice frame i want to get but it is optimised for 130mm forks. I’ll put the 120s on it then upgrade in time. Can anyone foresee any problems with this method?

You will be 10mm closer to the ground making crashing more comfortable.

What 130mm fork does the bike normally come with?
What 120mm fork do you have?

In certain cases different forks could be within a few mm of axle-crown length
eg Fox 32 @ 130mm QR* = 501mm
Rockshox Tora/Recon QR @ 120mm = 500mm
Revelation (not 2009 QR)/Pike @ 120mm = 498mm
Sector should be similar

*IIRC Fox 32 QR15 @ 130mm = 505mm

What frame is it?
It might be that you can get a taller/fatter lower headset cup to make up the extra height

Dependant on what the fork is you might be able to increase the travel on it also.

I was thinking of putting my rock shox tora in a ragley blue pig, but ive been looking at other frames and reckon i’ll just get one that suited to the forks i’ve got. Theres some nice ones i’ve seen, just need to do a bucks fizz and make my mind up.

Ragley Piglet?

Toras (same height as recons) ought to be able to be made to 130mm. assuming they’re air? Then they’d e 510mm, 1mm shorter than a Fox 32 QR @ 140mm