Sorry, no. That is not an option.

First you cannot convert an on-demand instance that includes charges for the content of the image into a BYOL instance for that content. SSsecond there is no in-place upgrade from RHEL to SLES.

You may consider machinery [1] [2] to help you build an equivalent system based on SLES.

HTH,
Robert

[1] http://machinery-project.org/
[2] https://www.suse.com/c/comparing-2-s...ing-machinery/