Debian system drive upgrade: copy filesystems from MBR partitions to GPT?

"Answers are: yes and yes. When you are copying smaller partition to larger GParted is basically doing the same thing: dd'ing old contents and launching resize2fs for the target partition. resize2fs requires some nasty sector computations so I'd recommend you to use (g)parted utilities which do it automatically, but it is of course possible with two mentioned commands. Also parteds do some verifications and sanity checks that won't allow you shoot yourself in the leg.

GPT migration has to do nothing with partition contents: partition table no matter is it GPT or MBR just instructs kernel how to position read/write window for partition device file; it does not affect contents of the partition at all."