Carry on replying yourself, you don't need a solicitor.
To answer the original question:
Nothing is set in stone. If there is a good reason to change the arrangements in the order then he is free to ask you to do so (as you are him). You are free to say no (as he is). If it's important and the refusal to change the arrangements is unreasonable, either of you could apply to vary the order in relation to that specific date.
eg. a close family member is getting married and you won't change the weekends around with 6 months notice - this would be seen as unreasonable by you and if he applied you'd lose.
eg. a close family member is getting married and you won't change the arrangements because he's only given you 1 week's notice and you have important plans on that weekend - he'd be seen as unreasonable but court would weigh up what's in DC's best interests, for you to cancel your plans or for DC to miss close relative's wedding.
eg. he wants to swap holiday dates, for understandable reasons - he has a new wife and SDCs and the dates have to suit all of them, the holiday is being taken with other family members who can only go that week etc. If he gives you plenty of notice then, again, his request would be seen as unreasonable. If he gives you very little notice, and you've made other commitments, then he may win or he may lose (depending on the precise circumstances).
You need to make the right "noises" here. Say you agree that a certain amount of flexibility should be expected from both of you, but when plenty of notice can be given then it should be, and neither of you should request or expect changes unless the reason to ask for them is sound, and no last minute changes should be requested unless there is a compelling reason or an emergency. Making last minute requests or demands is counter-productive because it puts am unnecessary strain on your relationship as co-parents and it also undermines the purpose of the order which is that everyone, most of all the DCs, know where they stand.
Unless there's a compelling reason to change, you'd be perfectly reasonable refusing. Explain that had he given notice you'd have considered agreeing, but as it is very little notice has been given (even though he must have known some time ago) and you have made your own arrangements which now cannot be undone.
If the frequency of the letters, and the reasons for them, are getting silly then I think you should also include this:
The order of x date was supposed to impose a regime of contact/residence so that all parties and the DCs knew where they stood and could plan accordingly. It was meant to remove the potential conflict caused by last minute requests/arrangements.
Your client has now instructed you to write to me x times in x weeks/months about relatively minor matters, and in tones which I find confrontational and distressing. This is creating unnecessary conflict and stress. I believe that your client is acting in the manner deliberately and that he is using you as a means of subjecting me to behaviour which amounts to harassment under the Protection from Harassment Act. Please speak to your client and explain that he cannot forever use legal channels to micromanage the time the children spend with each of us and the activities they do. Your client has a channel of communication with me (email) which he should use without relying on solicitors at every turn.
ONLY write this if he really is harassing you, otherwise you end up being the one who looks hostile.