5 Levels to Go... Heavensward

[Frizze]

I'm sure my question is going to display how ignorant I am...

Is there a prompt in the game that lets you know it is time to upgrade to Heavensward so you can continue your adventures?

A direct prompt? Not exactly. If you havent registered HW yet, anything you do at level 50 will stop giving you exp - so thats a clue. But the quest series that takes you from the end of the original story into the beginning of the expansion includes a decently long series of level 50 quests. The closest you get to a straight prompt is at the end of the patch 2.55 quests(the last of those level 50 quests). At this point(keeping this as spoiler free as i can) you have proven your friendship to the people of ishgard, and if i remember right theres some dialogue from one of your allies about getting you entry into the town soon. If the expansion is already registered at this point, the next quest shows up immediately and you find yourself getting invited to the new lands(originally there were a few months between when the 2.55 quests were added and the expansion released, so if you did it at the time you did have to wait a bit). Today its generally advised to get the expansion shortly before you get your first level 50, that way you dont lose any exp(or get it earlier if you want to make an au ra character or see it on sale).