I don't know how many drives your Pegasus has, but the following is true for 4,6, or 8 drives.
A 4-drive RAID5 has 3 drives worth of data and 1 drive of parity. For 4TB drives this amounts to a 12TB LUN.
A 4-drive RAID10 has 2 drives worth of data and 2 drives of parity. For 4TB drives this amounts to an 8TB LUN.
You cannot convert a 12TB LUN to an 8TB LUN, the data blocks cannot all map to a smaller LUN.
You can go from RAID5 to RAID10, but they have to be the same size. If you had a 3-drive RAID5, you could convert that to a 4-drive RAID10 as they have the same number of data blocks. Every block in the RAID5 will map to a block in the RAID10. But it is not possible to convert any 4-drive RAID5 to a 4-drive RAID10, the same is true for 6 and 8 drive RAID5 LUNs.
Note that the math is different for 6 and 8 drive arrays, but the effect is the same.
If you want a RAID10 your option is to offload the data, delete the R5, create a new R10 and copy the data back.