Maximum specified plate voltage for the 811A is 1500 VDC (50% Duty) and for the 572B it is 2750 VDC. Exceeding the tube's maximum voltage rating could result in a plate to grid arc and grid destruction. If your amplifier operates at a plate voltage of less than the maximum allowable for an 811A you should be fine. It is also important to verify the maximum specified plate current and plate power dissipation for the 811A to make sure you will not exceed those ratings as well. Tube sockets, plate connectors as well as filament requirements are the same between the 811A and 572B. Mechanically it is a simple swap, however there are a few electrical variances that complicate things.
Here's a link to the data sheets for both tubes, take a look. http://www.svetlana.com/docs/tubeframe.html