80mm sewer pipe

Hi, first thread post ever, so I hope this goes okay. My question is my 20 year old house has 80mm sewer pipe running between the floor/ceiling of a double story house joins up to 100mm pipe at ground level. Have to move toilet by a couple of feet due to renovation. Have replaced with a dual flush toilet. Went to buy a 5 degree 80mm coupling to move toilet and told I cant get one and the sewer pipe should be 100mm. Should I replace my 80mm pipe with 100mm or try to undo the metal hangers and get my new angle that way. Am concerned that the old toilet had trouble flushing and maybe the new toilet especially on a 3 litre flush might have trouble if it is going into an 80mm pipe instead of 100mm. Thanks in advance from a computer illiterate old soul.

I'd upsize it to 100mm

Same. Besides that the 100mm gear costs nothing compared to the 80mm gear. It may have been a space issue?

Thank you for answering gentlemen, appreciate it. Don't know why they used 80mm, i'm assuming it wasnt the legal size 20 years ago. I will have to knock some more plasterboard off tomorrow to get right to it all. I don't know yet if it was a space issue in the brick cavity, but plenty of room in floor/ceiling cavity, will have to wait and see. Thanks. Oh and if anyway wants to sate my curiousity what was 80mm pipe used for anyway? Thanks.

Would have been quite legal to use it 20 yrs ago and still is, though under certain conditions and restrictions, which only warrant buying it by the metre rather than the length, high rise blokes seem to be the only ones using it now and even then it seems to be a rarity. As I said also the cost is incredible, around 5yrs ago I had to buy a length at a cost of around $60 a length compared to 100mm at around $25 a length.