We purchased a table with Duplo baseplates on top (activity table) for my son when he was 1 year old. He's now turning six and we would like to convert the Duplo top to the regular LEGO System top. is there a baseplate that has Duplo studs on the bottom (so it sticks and essentially covers the Duplo spaces) and regular LEGO System studs on the top (so that the table can be used with the regular LEGO System pieces. It seems like a shame to chuck the table.
To answer your question, no there is no such plate with DUPLO tubes on the bottom and LEGO studs on top. HOWEVER regular LEGO and DUPLO are completely compatible - as long as the regular LEGO bricks are at least 2x2 studs. Just cover the DUPLO table with regular LEGO bricks and you are done...(c;
You could actually use regular LEGO plates too, however the connection won't be as secure as with regular LEGO bricks as plates are much thinner so won't go all the way down to cover the LEGO studs. But they do fit, and their benefit is that they cover a much larger area so you would need less.
Here is an article and some visual examples of how LEGO and DUPLO fits together very nicely: http://thebrickblogger.com/2010/12/lego-duplo/
Unfortunatly this is problematic.
Duplo baseplates generally have flat-topped studs, these studs are incompatible with lego. Furthermore thin lego plates don't have enough depth to take up a duplo stud.
So you are looking at at least two layers for the conversion each at least one full leog brick thic. The lower layer would be duplo plates (one lego brick high) followed by a layer of regular lego bricks. You can get oversized lego bricks which would probablly help in making it less likely for your conversion layers to come apart.