There is another aspect as well you need to be careful of. An order can be "changed" by it not being followed - even if you didn't intend that. So for example if you or your exed breached a few times and it wasn't enforced that would be seen by the court to be an accepted change so you'd be unable to enforce later. Likewise if you and the ex agreed to swap say Thursdays for Wednesdays for a while and she then refused to change it back to Thursdays and you carried on accepting Wednesdays, that would have been seen to be accepting a change.
It's ok if you're ok with a change, but the danger is she could then stop the Wednesdays and you couldn't enforce them as they're not in the court order. So the usual advice is to make sure any temporary changes are agreed in writing, clearly and formally, followed by the phrase "and then the arrangements will revert to those within the court order".
Eg if you agreed to swap a court ordered weeks holiday for a different one, you would send an email saying "I agree to change week 1 to week 3 this summer, and then the arrangements would revert to those in the court order".
Advice is usually not to change anything at all without it being varied formally - eg if you both agree to change something in the order, get a solicitor to draw up a new consent order. You'd both need a solicitor for a one off job - one to draw up the new consent order, the other to approve the wording and then they send it to the court for sealing.
I fell foul of swapping a court ordered week's holiday in the summer once. Agreed it in writing and booked a holiday for the changed week. My ex then said son didn't want to come that week as she'd organised a play event for a week. I ended up having to do a specific issues order to get the holiday ordered and the weeks changed for that summer. It was a nasty trick because you can't enforce a swapped week.
If you're wanting to change an order to less time, you could just do that by agreement with the ex in writing - but then you wouldn't get that time back again even if it's in the order. If you wanted more time, or a different schedule, your ex is unlikely to agree, and then you'd be expected to try mediation first and then submit an application to vary.
Sometimes orders do need to change if someone moves house or job etc.
If wanting more time, it's unlikely to achieve anything applying for variation for more time as the court thinks the kids have an established routine. The only real reason for changing it to be more time is if the ex has done some really bad stuff and it's impacting on the child. Eg to change it from spends time with to lives with both plus an extra night - which effectively takes something away from her to deter her from certain behaviours or disruptions.
This is all based on legal advice given to me but I'm not a lawyer obviously. But never agree to change something informally - even 1 missed night - always confirm it in a short polite formal email adding "and then the arrangements will revert to those within the court order".
