Switching Products

[#1]

I recently changed over to the PLusnet Value broadband plan (approx £6 a month with 10GB usage cap), and that plan took effect on the 15th of March 2011. Last night I received an email that Im "approaching your usage allowance" so I figured this is not working for me; I logged into my account and changed products to the Plusnet Broadband product with 60GB usage limit for approx £11 a month.
I've received an email this morning that this change couldnt be applied.
Am i now locked into the £6 pm plan with 10GB limit for a year?
Thanks

[#2]

You can only make one product change in a billing month
I am not sure why you aren't allowed to change as this is the new billing month. Either phone or wait for a rep to pick it up on Monday

[#3]

Thank you.