Let it sure completely and then decide.

It can take a week or longer.

Don't panic, polyurethane finish is also a varnish. The beauty of varnish is that it should have more than one coat anyway. I'm not familiar with the P&L varnish. Is it thinned with turps or paint thinner? Varnish should be sanded lightly between coats any way. Anything finer than 150 will be OK, I use P-350 between varnish coats. Varnish is just resin dissolved in oil. Poly is just a synthetic resin, I don't know which resin P&L uses. It's probably on the label. The only question is do you prefer gloss, semi-, or matte? Just let each coat get hard before re-coating. I give mine 4-10 days. But I use "old-timey" varnishes. read label

Old Pratt & Lambert oil varnish was good stuff; I used a lot of it in the 80's, and their "Tonetic Wood Stains" too! Whatever happened in the ensuing years (formula changes) I don't know. If the poly had some degree of sanding prep, or was very recent it will probably be fine. If the poly was old and hard and had no sanding at all you could get peeling in the next few decades. Oil varnish sticks pretty well to just about anything. It will be as long as a year before the new varnish is dry enough to begin to crawl or peel.
Casey

A professional painter just told me that putting varnish over polyurethane would result in peeling (apparently due to differences in how the two materials expand/contract in response to direct sunlight). I bought spar varnish for improved UV protection. Are my UV protection efforts dead in the water?