I actually had a gold charge card prior to recently applying for plat charge for the 80k bonus and prior to 'upgrading', I called platinum services to see if they were willing to extend the 80k offer. I was told they wouldn't do that since I was an existing cardholder but due to my travel requirements, I decided to just apply anyway without expecting much. I was told I'd have to apply online.
When I filled out the application form, I noted that there was no option to upgrade from the gold charge to the plat charge (if there was one, I certainly missed it) but it did have me put in my existing amex number in there.
Got the card a few weeks ago and on my first week, I charged around $2k on it and was quite surprised to see the 80k bonus point posted.
What I noticed though was the plat charge's MR points initially went into a separate pool compared to my gold charge so I had to have them manually move the gold charge points across to the plat charge. This was a bit unusual for me because all of the other cards attached (i.e. the plat reserve) was correctly listed as a child card under the plat charge in terms of MR.
Given the above, I suspect it all depends on how MR gets setup on your account.