I've been active on the "SAMD Cortex-M0+" thread of the forum for the last two years, as I did dev on my project, using a SAMD20J18. It was always going to be a squeeze, but after repeatedly running out of RAM space (which I reconfigured for) I have now run out of ROM space... with code still to write!
We had already decided to move to a bigger device some time back, but now it's a fact.

I'm starting with the porting of my code today, as soon as I get Atmel Studio 7 installed. Hopefully it should be doable without any tears! Any pointers of possible pit-falls would be helpful!

Go well.
Sarlacii