How late is to late to cancel a private appointment with a Professional.

[Theydonotcare]

NC'd
Posting for traffic.
I have an important private appointment booked for December.
I panicked ATT DD is having serious issues in preschool. We are waiting on early intervention and on the waiting list 16 months for an assessment.
As things are getting worse I booked a private assessment for December.
The issue is I can pay it but it means a really tight Christmas no fun outing or nice gifts for the other DC over Christmas.
I don't want to let the assessment lady down. How bad would it be if I asked her to defer until January.
January is a good month for us we work over Christmas lots of double time etc but bank holiday days.

I know when you're involved in something it seems like the biggest problem in the world but, in the nicest way possible, you are making way to big a deal about changing the appointment. Just call and ask, 'Can we change * appointment to January please?'. That's it. No repercussions or explanation necessary. Just call, right now.