Scatty how long have you worked there - only since April or longer than a year?
First point to make is that if they are trying to save money I would be surprised if they want to move you all on to normal contracts as presumably that would mean more hours.
However to answer the question about whether they can amend your contract or not, the short answer is that the basic principle is no, you have to agree it. The long answer is it's a bit more complicated than that, and as long as there is a good business reason why they need to do it and they are prepared to jump through several hoops, they probably can do it.
See here about changing terms and conditions and here about what happens if you don't agree. As you can see, if they are confident there is a good business reason to do it, and willing to risk that you won't bring a difficult-to-win unfair dismissal case, they can do it if they need to.
Although the principle of not changing contracts without consent of both parties is there and is a good one, I'm sure you will appreciate that there are times when an employer simply has to alter terms and conditions, has no choice, and there has to therefore be a mechanism whereby they can do that.
Read the links I have sent you so you are fully informed and go to the meeting and see what happens. Do you have any particular reason to think they want to put you on a normal contract - have you had any other indications that this is what they want? If you have a union rep going to the meeting try and relax a bit - hopefully he/she knows his/her stuff.