I have no experience with spunding, but I am a physicist and I have experience with cooling beer and transferring carbonated beer.
There are 3 factors that affect the pressure when cooling beer from ale fermentation temperature to near freezing: 1) the shrinkage of the liquid volume (look up water density vs temperature), 2) the shrinkage of the gas volume in the head space according to PV=NRT, and 3) the solubility of CO2 as a function of temperature. The first one is minor. The pressure change from the second one is significant, and the third is a function of the ratio of liquid to head space volume. CO2 solubility increases as you cool the beer, so the beer dissolves gas from the head space and reduces the pressure. My experience is with using a mylar balloon to supply the CO2 to keep the pressure from going negative as the beer cools. but I would be very surprised if the pressure drop would exceed 25 psi in any reasonable temperature drop and liquid/gas ratio. When starting with 25 psi at 20 C I would expect that you would still end up with positive pressure even when cooling to near 0 C.
Now for the foaming issue. My experience is with using a beer gun to transfer carbonated beer from a pressurized keg into bottles. The first step is to drop the head pressure to a few psi for the transfer. The cooling will help with that, but you will probably still need to open a pressure relief valve to lower the pressure even more. That will not affect the carbonation level in the beer in the short run, but will slow the transfer rate. To deal with the carbonation of the beer you need a long transfer line with a small inner diameter so there is a large pressure drop along the line, so the beer will emerge without foaming. It is not much different than balancing a draft system, but your end goal is no foam instead of a desirable amount of foam. Using 10 feet of a small diameter tubing should be enough. The transfer will take a long time with low head pressure and a high impedance line, but that is the price you need to pay to eliminate foaming.
To repeat, I have no experience with spunding, so I am extrapolating from what I do know.