A couple of questions...How long has it been since your atf was changed and your problem showed up? Also, are the fluid levels correct? Too low a level and you have a possible slippage condition. Too high a level and the fluid becomes "frothed", which is air is being introduced into the fluid, causing slippage problems.
Synthetic fluids do not cause seals to swell and then contract. Synthetics contain additives that provide lubrication qualities, thermal stabilizing properties, anti-wear, etc. You might develop leaks after leaving non-synthetic atf because synthetic is non-evaporative, which means that the "regular" is leaving a coke (a deposit or residue) at the seal area which helps maintain the seal integrity, whereas the synthetic will "clean" that area, causing the defective seal to show its true colors.
One major question that also needs to be addressed: Was the fluid the appropriate standard (such as Dextron III, Mercon V, etc.)? Thicker or thinner does not matter, what is right for it does, though.
One last problem, your old fluid may have been contaminated with clutch/disc material from normal wear and tear. By removing the contaminated fluid and adding new fluid you may have removed part of the friction that was making your transmission operate normally.
So, first check you fluid level at operating temperatures, then find out what the fluid standard used is, then decide if you want to drive it as is, sell it, or take it to a repair shop for further analysis.