Some years ago i had some issues while doing homogeneous system copies (ECC 6.0 SR3+MaxDB 7.x) when i didn't use the same kernel, but i guess it was maybe because the kernel included with the Installation Master at that time was in a lower patch level than the one i had on the landscape. 
I will patch to 7.20 before using the latest SWPM, then after migration i will update to 7.21 EXT.
Thanks for the help, i really appreciate it. 
Best regards.