I would add three months on to your current holiday year and give everyone three months extra entitlement now, so they can plan their holidays accordingly.

I have done it a few times , all at the time for seemingly good reasons, rarely has it made a huge difference.

But as Jackie says do a 15 month holiday year in year one. You might need to put some rules around when colleagues can take leave (not all in first quarter etc)

I did this at the start of 2022, the leave year was changed from January-Dec to Sept-August. We got everyone's agreement to do this.

We use Breathe HR and there wasn't an option to change the leave year to a 16 month year so I had to calculate what everyone would have carried over into September 2021 and then made sure everyone had the correct allowance when the leave year was changed retrospectively. It was a headache but luckily we have less than 40 employees so it didn't take too long.

If I was in your position I would do a 15 month leave year from 1st October 2022 if your HR systems allow.

We have swapped couple of times. Just add the extra leaves to employee total and allow them to carry over for the next year if needed.