Obviously ring SFW to check for yourself. They had a policy change 2 years ago, so what I say below maybe the 'old' system.
I believe in Wales, OU is classed as part time studying. Part time funding and full time funding are different.
So if you move from part time funding to full time funding, you get the full entitlement for money and the funding for the full amount of years. Which is a max of 4 years, 3 years of studying for a degree with 1 gift year incase you fail 1 year.
This works to your benefit in costs because full time cost is MUCH higher than part time costs. Having 120 credits you can 'miss' 1 year of funding and go straight into the second year of a brick university. 1 year less of full time debt.
However Universities have very strict rules for moving straight into the second year, basically the 120 credits you have already must match in content, the brick uni's first year of 120 credits.