The problem will be that Unraid uses a different start sector for HDD and SSD (for performance reasons) so the SSD will have a few less sectors available for data than the HDD had.
The only easy way around this that I can think off is to use the New Config tool to get the SSD into the array, rebuild parity, format the SSD, and then mount the old HDD drive via Unassigned Devices to copy its data back to the array. This would not be ideal as if the HDD failed during this process (and you do not have a backup of its contents) you could end up with data loss.