Am wanting to migrate slowly.. by domain from 15x to 17x,  (We have two licenses)

Right now we have this configuration

Inbound mail servers -->SM15.x --> and to SM17.x for other domains....
So was thinking that we could move one domain at time from SM17x.

How I was planning to do that was...

Internal...

a.) set up domain on Sm17
--> make sure DKIM matches
--> (no need to change SPF because outbound email servers
       -which are different - doesn't change)
b.) Copy over the directories of said "domain" .. to SM17x.
c.) change Sm15x  "domain" to external - using Host adderess

d.) Run file-sync one more time between two domain directories

External (we have our 4 sets of DNS)
1.) Day before - change TTL to 1 hr for "domain"

2.) After step c.) change the the domain/sub-domain, that the users
     use = to access SMTP/POP to new server IP

3.) Change auto-discovery.

not to keen to do big-bang.. but maybe the above solution is too risky too???
Opinions , thoughts, recommendations???