I had the same problem in my first 928, but if it is blowing hot that means the hot water is getting where it should, but you cannot turn it off or control it. So why would you need to change hoses? Sounds a bit strange.
The vacuum system is a pain as any leak anywhere will stop the heater, cruise control and a few other things working as they are all linked together.
The best thing I did was to buy a vacuum tester, they are fairly cheap and are so helpful to pin point any leaks and check the valves work. They can be got from Machine mart ( https://www.machinemart.co.uk/shop/product/details/gunson-77003-vacuum-pump?da=1&TC=SRC-vacuum) and online. An alternative method is to pull the vacuum pipe out of the valve start the car and put your finger on the end and see if there is vacuum. This can be done for all the pipes.
The heater valve on the bulkhead under the air filter can be cable-tied shut which would enable you to see if this is the problem and you have a vacuum leak.
I had two of the vacuum valves in the dash leaking, a pain to get to as I had to remove the centre consul, but I managed to block them off and this then gave hot or cold. This was as far as I got before I sold that car.
Other than the vacuum side of things there are a few other checks you can do on the bulk head on the passenger side the is a vacuum connecter called the spider that has several pipes coming in and out of it, I have been told that these can be troublesome so check all the pipes are seated well. Also all of the black rubber connecters need checking to see if the pipes are loose so losing vacuum.
If you lie upside down with your head in the drivers foot well and look up there is a little black box (motor) with a linkage that comes out of the side. It is at the back and slightly to the left if I remember correctly. This is the unit that mixes hot and cold air. If this doesn't move when you move the temperature slide obviously you will only get hot air. This can be pushed one way or the other if it is not working to give you cold.
The other checks are both the internal temperature sensor, located on the dash, and the external temperature sensor located in the front passenger side wing under the light pod. Most people who I have talked to have often found this to be faulty, but if I remember correctly if they fail it usually means you get hot or cold and nothing in between.
There are links on how to get to this and other checks you can do on this site http://www.nichols.nu/tips.
Hope this helps a bit. It is all I can remember doing when I went over mine
] first before i rip things out
