One more option apart from refreshing the SFR is to use t he "H" version of the 89c51rd2. Here the switching between 6 and 12 clock is not the same as in non H version. I am not sure if you are talking of a single application or volume production - in which case the "H" option may or may not solve the issue. Read for some more on this here : http://www.8052.com/forum/read.phtml?id=63076

And Prahlad says that 89c51rdxx is being replaced by 89v51rdxx - in which case better to switch over now itself when the project has not settled down ?

Raghu