I have SMC2108 (LSI controller) hardware controller, which I want to replace with LSI SAS 9207-4i4e (it's HBA - fakeraid). Can I switch the controllers and to access my data (even with readonly). I know that SMC2108 keeps the metadata in the DDF format in each disk, and mdadm have support of DDF. So anyone have actually done such transition from hardware LSI (DDF superblock format) to software mdadm? In fact I only need to read my data and then I will recreate ordinary software raid with mdadm.
You asked about two different targets (hardware card or software mdadm) so I'll answer them each separately.
As for moving between cards, going from LSI to LSI usually works great. In my experience, transplanting an entire array from an older series LSI card to a newer 9620 with absolutely no hiccups. The controllers understand the metadata well enough and import the correct configuration. If it doesn't import the configuration correctly, just back out of the BIOS tool without making any changes, and plug the old card in.
If you have 50% redundancy in your array (e.g. a 2 disk RAID1), it can't hurt to take one disk out and plug it into the new card. The system should pick up the configuration from this disk alone. If you are able to boot into it and see data, you are all set. Just add the other disk to the new card too, and let it rebuild.
As for software raid, depending on the number of disks and the type of RAID, LSI's setup allows you to mount disks directly from command line in linux. I've taken apart a RAID1 from an LSI 9620 (identical to your SMC2108), plugged one disk right into the motherboard, and booted. If you have a RAID5 or RAID10, obviously that would not work quite so well.
A best course of action would be to use extra hard disks to make your mdadm raid in the correct size and configuration, then copy data over.