While Apple has increased the size of the Apple Watch over the years, users have always been able to use older watch bands with newer models. That might soon change. Initially spotted by MacRumors, leaker UnclePan claims that the so-called Pro — Apple’s forthcoming rugged smartwatch — may not be compatible with existing straps.

Until now, Apple’s been able to easily maintain compatibility because the Apple Watch’s design has barely changed. Each Apple Watch model comes in two sizes: a smaller version and one that’s 4mm larger. The original watch up through the Series 3, for example, was sold in 38mm and 42mm sizes. That changed with the Series 4, which increased the cases to 40mm and 44mm. Apple once again bumped up the size with the Series 7 to 41mm and 45mm. That’s 3mm over seven years — and not once did that warrant users needing to buy new straps unless they were moving from the smaller watch to the larger one.

Bloomberg’s Mark Gurman has said that the Pro will be a “good bit bigger than the standard Apple Watch” and may only appeal to a certain niche. So far, it seems the rumor mill has settled on the Pro having a two-inch display housed in a 47 mm case. If this pans out, that larger display could be the culprit. It’s admittedly hard to see straps made for the 38mm / 40mm / 41mm Apple Watches working on a 47mm watch. That’s a significant 6-9mm increase.

Other design tweaks could also play a role. Gurman also noted that the Pro will sport a fresh look that’s an “evolution of the current rectangular shape.” Perhaps this “evolution” meant Apple had to redesign how the straps attach to the case for a more secure fit or added durability.

That said, it isn’t clear whether everyone upgrading to the Pro would need new straps. It’s very possible that you may only need to switch if you’re upgrading from the smaller model to the Pro. If you have a 42mm / 44mm / 45 Apple Watch, the case size is only increasing 2-5mm — which isn’t too far off from past increases. It, as Gurman suggests, just might not look as nice or fit quite as well.

In any case, you should take this with a grain of salt. (Remember the flat edges redesign rumored for the Series 7 that never came to pass?) That said, this is a new wearable category, and it wouldn’t be surprising if Apple decided to make a clear break with watches that came before the Pro. The real takeaway from all these rumors is Apple’s gearing up to make some dramatic changes to its watch lineup — and we’ll find out exactly what those changes are at next week’s “Far out” event.