Our company is planning to change our primary domain.
We followed this thread - https://nation.marketo.com/message/120363#comment-120363
One comment in the thread stated: "What are you doing about your tracking cookies? Major question if you want to keep lead activity history."
The Marketo product docs states, "Cookies are not shareable across domains."
Do we lose all the activity history on a record when we change a domain?
Correct. Without significant JS development effort, the old session associations will be gone and everyone will begin again as anonymous.
Public documentation, no.
It's the kind of thing done by Marketo-specialized JS developers and the code is complex enough to be kept close to the vest. (All follows logically, though, if you see all the nooks and crannies.) Bad karma to estimate other people's numbers... but ~15-20 hours to implement & test.
Yes, Cookies are not sharable in between different domains. Marketo uses a tracking cookie names "_mkto_trk" to identify either lead in known or unknown. If the domain is changed then this cookie will be regenerated by the Maketo as per the new domain. Hence all leads will be considered as anonymous.