When do you really need API versioning for internal services?

We're building up our internal service mesh, and honestly, we've just been pushing the newest stuff, coordinating all the changes across our different teams. It's been okay so far, but I have a feeling this isn't going to work much longer without causing some real headaches. At what point do you all think API versioning, even for stuff that's only used inside, becomes super important for handling dependencies and making sure new things roll out easily? Is it just about how many teams you have, or how many services, or maybe how fast those main APIs are changing?