Bathroom fitting

As long as your boiler is not in your bathroom it will not affect your pipe work in your bathroom.

Should be completely fine till the existing boiler is not in the bathroom

I think the answer is more nuanced. Firstly you mention your boiler is conventional, obviously I have not seen it, but I am going to cover a few bases. Secondly, if by conventional, you mean your boiler is heating a gravity-fed system, i.e. you have a cylinder in a cupboard or bathroom, and this is located upstairs, then it will depend on where you locate the new boiler. If it downstairs, then that will need connecting to the main flow and return pipes, which would likely be running under the floor of the landing at the top of the stairs. If the bathroom is above the location of the boiler, then it is likely to affect the bathroom. I say this because a conventional boiler can sometimes be heating a cylinder where the water is stored. A new combi has more pipework as it removes the stored element of the system. The main things to consider as to whether it will or will not affect the bathroom is: Where the new boiler is to be cited? What is above/below this location.?(Some boilers are fitted in loft spaces) What type of system do you have presently? (Is there a cylinder, if so where) It can affect the bathroom, and I have seen the implications of this when the boiler is under the bathroom, the plumbers will need to connect to the existing pipework for the flow and returns. The last job I saw this on they surface-clipped the pipework through the bathroom and through the bedroom next door, due to the flow and return running down the chimney breast. I would perhaps consider getting the boiler done first to avoid any doubt.