How to switch sd cards when using a data2whatever (d2w) custom ROM?
AFAIK the different partitions are identified by their ordering on the SD card. Therefore the standard FAT32 partition comes first, followed by the app/data partition (usually ext3 or ext4). Sometimes a swap partition follows as third partition.

If you use the same partition pattern (number of partitions and partition types) on the new sd-card there should be no problem. The partition sizes should not play a role - as long as you don't reduce the size.
Just partition the sd-card, format the partitions and copy your data from your old sd-card over to the new.