Now that thin to cache conversion is allowed, the following warning error isn't technically true. This command does accept LV type thin, but just not for writecache. 
"WARNING: Command on LV VG/thin_lv does not accept LV type thin."

lvm2-2.03.23-2.el9    BUILT: Sat Feb  3 01:10:34 AM CET 2024
lvm2-libs-2.03.23-2.el9    BUILT: Sat Feb  3 01:10:34 AM CET 2024



[root@grant-01 ~]# lvcreate --yes --type thin-pool -n thin_pool  -L 34G thinpool_sanity  
  Thin pool volume with chunk size 64.00 KiB can address at most <15.88 TiB of data.
  Logical volume "thin_pool" created.

[root@grant-01 ~]# lvcreate --yes -n thin_lv  -V 200G thinpool_sanity/thin_pool 
  WARNING: Sum of all thin volume sizes (200.00 GiB) exceeds the size of thin pool thinpool_sanity/thin_pool (34.00 GiB).
  WARNING: You have not turned on protection against thin pools running out of space.
  Logical volume "thin_lv" created.

[root@grant-01 ~]# lvcreate --yes --type linear -n to-cpool  -L 2G thinpool_sanity  
  WARNING: Sum of all thin volume sizes (200.00 GiB) exceeds the size of thin pools (34.00 GiB).
  WARNING: You have not turned on protection against thin pools running out of space.
  WARNING: Set activation/thin_pool_autoextend_threshold below 100 to trigger automatic extension of thin pools before they get full.
  Logical volume "to-cpool" created.

[root@grant-01 ~]# lvs -a -o +devices
  LV                VG              Attr       LSize   Pool      Origin Data%  Meta%  Move Log Cpy%Sync Convert Devices           
  [lvol0_pmspare]   thinpool_sanity ewi-------  36.00m                                                          /dev/sdb1(0)      
  thin_lv           thinpool_sanity Vwi-a-tz-- 200.00g thin_pool        0.00                                                      
  thin_pool         thinpool_sanity twi-aotz--  34.00g                  0.00   10.47                            thin_pool_tdata(0)
  [thin_pool_tdata] thinpool_sanity Twi-ao----  34.00g                                                          /dev/sdb1(9)      
  [thin_pool_tmeta] thinpool_sanity ewi-ao----  36.00m                                                          /dev/nvme1n1p1(0) 
  to-cpool          thinpool_sanity -wi-a-----   2.00g                                                          /dev/sdb1(8713)   

# to write cache attempt
[root@grant-01 ~]# lvconvert --yes --type writecache --cachevol thinpool_sanity/to-cpool thinpool_sanity/thin_lv
  WARNING: Command on LV thinpool_sanity/thin_lv does not accept LV type thin.
  Command not permitted on LV thinpool_sanity/thin_lv.

# to cache attempt
[root@grant-01 ~]# lvconvert --yes --type cache --cachepool thinpool_sanity/to-cpool thinpool_sanity/thin_lv
  WARNING: Converting thinpool_sanity/to-cpool to cache pool's data volume with metadata wiping.
  THIS WILL DESTROY CONTENT OF LOGICAL VOLUME (filesystem etc.)
  WARNING: Sum of all thin volume sizes (200.00 GiB) exceeds the size of thin pools (34.00 GiB).
  WARNING: You have not turned on protection against thin pools running out of space.
  WARNING: Set activation/thin_pool_autoextend_threshold below 100 to trigger automatic extension of thin pools before they get full.
  Converted thinpool_sanity/to-cpool to cache pool.
  Logical volume thinpool_sanity/thin_lv is now cached.