This should work in theory.
Net up your new crypt pointing to a new directory that isn't a subdirectory of the current crypt (that's important!). Make its setting the same except for the filename encryption.
Use the --server-side-across-configs flag and something like
rclone move -v oldcrypt: newcrypt: --server-side-across-configs
Here is a test I did which worked
[oldcrypt]
type = crypt
remote = dropbox:oldcrypt
filename_encryption = standard
password = XXX
[newcrypt]
type = crypt
remote = dropbox:newcrypt
filename_encryption = standard
password = XXX
filename_encoding = base32768
I uploaded a single test file
echo hello | rclone rcat -vv oldcrypt:hello.txt
Then server side moved it
$ rclone move --server-side-across-configs -vv oldcrypt: newcrypt:
2023/04/18 11:22:45 DEBUG : rclone: Version "v1.63.0-beta.6959.b87a9c835.fix-listr-performance" starting with parameters ["rclone" "move" "--server-side-across-configs" "-vv" "oldcrypt:" "newcrypt:"]
2023/04/18 11:22:45 DEBUG : Creating backend with remote "oldcrypt:"
2023/04/18 11:22:45 DEBUG : Using config file from "/home/ncw/.rclone.conf"
2023/04/18 11:22:45 DEBUG : Creating backend with remote "dropbox:oldcrypt"
2023/04/18 11:22:45 DEBUG : Creating backend with remote "newcrypt:"
2023/04/18 11:22:45 DEBUG : Creating backend with remote "dropbox:newcrypt"
2023/04/18 11:22:46 DEBUG : hello.txt: Need to transfer - File not found at Destination
2023/04/18 11:22:46 DEBUG : Encrypted drive 'newcrypt:': Waiting for checks to finish
2023/04/18 11:22:46 DEBUG : Encrypted drive 'newcrypt:': Waiting for transfers to finish
2023/04/18 11:22:47 INFO : hello.txt: Moved (server-side)
2023/04/18 11:22:47 INFO : There was nothing to transfer
2023/04/18 11:22:47 INFO :
Transferred: 6 B / 6 B, 100%, 0 B/s, ETA -
Checks: 1 / 1, 100%
Renamed: 1
Elapsed time: 2.3s
2023/04/18 11:22:47 DEBUG : 7 go routines active
2023/04/18 11:22:47 INFO : Dropbox root 'oldcrypt': Committing uploads - please wait...
2023/04/18 11:22:47 INFO : Dropbox root 'newcrypt': Committing uploads - please wait...
Note the Moved (server-side) which means it was indeed a server side move.
And the file arrived safely
$ rclone cat -vv newcrypt:hello.txt
2023/04/18 11:22:51 DEBUG : rclone: Version "v1.63.0-beta.6959.b87a9c835.fix-listr-performance" starting with parameters ["rclone" "cat" "-vv" "newcrypt:hello.txt"]
2023/04/18 11:22:51 DEBUG : Creating backend with remote "newcrypt:hello.txt"
2023/04/18 11:22:51 DEBUG : Using config file from "/home/ncw/.rclone.conf"
2023/04/18 11:22:51 DEBUG : Creating backend with remote "dropbox:newcrypt/覽㕵慻Ꙁ甽ހ作騐跟"
2023/04/18 11:22:52 DEBUG : fs cache: adding new entry for parent of "dropbox:newcrypt/覽㕵慻Ꙁ甽ހ作騐跟", "dropbox:newcrypt"
hello
2023/04/18 11:22:53 DEBUG : 9 go routines active
2023/04/18 11:22:53 INFO : Dropbox root 'newcrypt': Committing uploads - please wait...