Central Heating Pipes

I am about to put down an wood floor and need to sort out the pipes underneath first.
My problem is that I have a combi boiler with 22mm flow/return pipes. However, whoever fitted the systemhas basically welded 15mm pipes on. The result is that the pipe is too narrow to allow hot water to flow out quickly enough with the result that the boiler is constantly switching itself off and on. So, my plan is to replace the 'backbone' of the central heating system with 22mm with 15mm for each spur.
I am about to put a wood floor down in my dining room and need to sort out the pipes before I do. The problem is that the pipe leaves the boiler runs under the kitchen floor (where it is embedded in concrete) before it gets to the dining room. So, I have no access to the pipe under the kitchen floor unless I take up the floor which I am not planning on doing just yet.
Would it be ok to attach 22mm pipes to the 15mm pipe where it emerges from under the kitchen floor and, later on, dig up the kitchen floor and replace the remaining section of 15mm piping?
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