Yup be careful of this it happened to me last 2 xpac's. I used to main Resto Druid, I went Feral Tank for 1 raid and bam stuck tanking through the expansion. Then all of a sudden its Mop I come in on my Ret pally, go prot 1 raid and bam stuck prot until last week when we found another tank.
Right but if you really didn't want to go Prot until you found a new tank, you could do the alternative which is what you would be doing anyway if you didn't go Prot: Find a new guild where you could just play Ret.
At least your guild was accepting that you didn't want to play Prot so they found a new tank, instead of telling you that you were the new tank.