Multi-Client Simulation dummies don't have IDs

I apologize for the ambiguous title, but I don’t how to clearly ask this question without context, so here it goes:
I’m working on this tycoon game (I figured it’d be a good starter project), and when a player touches a part to claim a plot of land, their user ID is saved and is used for making sure that only the owner can interact with their tycoon.
And here is where my problem is. I want to playtest my game with multiple players. One reason being that I want to eventually implement a feature where when the owner of a tycoon leaves the game, their tycoon is reset for the next player to use it. Except the problem is that the dummies in the test servers don’t have user IDs. The initial reason why I used IDs was because I figured that if I ever wanted to save a player’s tycoon state for the next time they play, I could just save the tycoon’s state to their ID, since their ID wouldn’t have changed. In hindsight, I would’ve just used their username instead, but still save their tycoon to their ID. But due to the way that I’ve designed the game up to this point, I feel like it’s too late.
At where I am right now, I would really hate to go back into every script and change it so that their username is being used instead (and also because I fear that doing so would risk breaking something in the game).
I would really appreciate any solutions or recommendations!

I ran into this problem as well.

The only way I found around it was by having my friends playtest it with me in a live server. Sorta sucks.