Hi!
With due respect 'Petre Pircalabu'. I think you do not have any previous experience with 8031/51 series.
Let me tell you that i have no idea about "XS40 Board". but I know one thing you can replace 8031 (with your eyes closed) with 8051 or 8751. If i got some board with 8031 and if i want to change the behavour of 8031 then i will replace it with 8751. (Except that the need of ROM will not be greater then 8751's built in ROM). Even if board does not permit you to change EA pin level. I would pull that pin (EA) up and sold it to make it use internal memory.