Advice to upgrade 2014R2 to 2018 incl. Messenger

Greetings,

we're running GW 2014R2 incl Messenger 3 on OES 2015 SP1 (NSS Volume, 1 PO, GWIA, Webaccess)

As Messenger 3 is the only programm that requires eDir authentification we want to upgrade to GW 18 incl Messenger 18.

As far as I understand the docs updating GW to 18 is no big deal on OES2015SP1.

But for messenger 18 the docs stated that SLES12 is required. (OES2015 is on SLES11).

How is the best way to upgrade to GW 18 incl. Messenger 18?
Our GMS 2014R2 is already on a single machine with SLES12SP3
Should we use another SLES12 machine for Messenger?