Thank you for your question
My name is Clare
I shall do my best to help you but I need some further information first
What has made her decide she wants this?
How old are the children?
My apologies for the delay
Neither of you can simply unilaterally change the arrangements - it is something that has to be negotiated and agreed - or for a court to decide if the pair of you cannot do so.
It is not reasonable for her to try and impose a pattern that does not match your work responsibilities and if the matter did go to court this would not be ordered - not least because it means that you would never get a full weekend with the children.
You should try and discuss matters with her using Family mediation
but if mediation fails then you should apply to the court for a Child arrangement Order.
Hopefully once your ex realises that she cannot succeed then it will be possible to revert to the flexible arrangements you have had until now.
I hope that this is of assistance - please ask if you need further details
YOU must attend a Mediation Information and Assessment meeting before you can apply to the court - so it is worth a try