Hello, my name is Ben and it is my pleasure to assist you with your question today.
I understand you are saying that you are not starting a new job, but legally you are because your current job is part time and the new job would be full time. It is not just the nature of the job or job title that defines a role, there are many other terms that are relevant too and the same duties part time would be considered a different job/position that the same duties being don full time. So it is possible for an employer to offer you a new contract for full time hours but on new terms to what you do now.
The main issue here would be whether you are being treated detrimentally because you are on maternity leave or not. For example, would an employee in the same position as you but not on maternity leave have been treated better than you in the circumstances. However, if this is a blanket policy on all new joiners/promotions since 1 April, then this is applied equally to all and not really discriminatory. So this is really a matter of accepting the new contract as it is or trying to negotiate some changes or refusing to do so and remaining on the current one.
Hello, I see you have accessed and read my answer to your query. Please let me know if this has answered your original question or if you need me to clarify anything else for you in relation to this?