Chance of downgrade would be incredibly low. In the very rare event that it happened, it would happen on the day. As far as I am aware there is no evidence of a paid seat selection making you less likely to be downgraded.

I would save the cash and check in at exactly 24hours. I would use some of the money to pay for an EF subscription and put seat alerts on the return sector so if 2 seats are released in U Class you get alerted straight away and can make the swap.