Your (non-RAID) data-disks will be fine, data-loss is not supposed to happen.
AHCI vs RAID vs IDE/Legacy mode doesn't have any relevance for the content of the disk. The disk should report the same capacity, block-numbering scheme, etc. to the BIOS and the OS.
The only difference is in the device-type (and capabilities) of the disk-controller that the BIOS will present to the OS.
I've done dozens of such conversions (I lost count, could be well over 100). Never seen any issue.