Depending of your current running services, it can be straightforward. Lot of tools are available between these two platforms.
backup your server, jails and every data. If you are using zfs, you can use
zfs snapshot with
zfs send and
zfs receive. If you don't have ZFS, and use standard FreeBSD filesystem, you can use
mksnap_ffs. I hope you have decent solution to backup your data, like bacula, little hard to use the first time, but really useful if you make mistakes!
list all your services on FreeBSD server or FreeBSD jails (e.g.
service -le), don't forget to migrate cronjob (all crontabs are stored in
/var/cron/tabs on FreeBSD) and
at job if you are using it.
try to find same service working on RHEL server, you can use
yum search, freshports and pbone. Be careful with this step, FreeBSD has recent version of software, RHEL freeze its package collection, read release notes before doing something wrong and test configuration compatibility.
make your test platform with all these information.
test your services.
read logs. If something go wrong, read FreeBSD and RHEL documentation concerning your issue.
I don't know how your platform is configured currently, but, if all data (customer, backend...) are remotely stored on SAN or NAS (e.g. NFS, iSCSI...), it can be really easy to migrate. In this particular case, only configuration on FreeBSD and RHEL server will be modified.
Last advice: read old log and try to find working/not working pattern on it and replay it on your new testing platform.