Anyone having issues modifying their mobile plans before Introductory plan starts?

Never handled this type of situation maybe fizz can help you on that or other knowledgeable experts on the forum

If actually no change was made to your plan then you need to contact Fizz.

As suggested by @Ed, its better to check the price and how much Fizz charged on the credit card after changing plan

I changed my plan minutes after getting the email about my beta period ending and I was able to do it, but then I got locked out for the next two days just like you. But there was another thread yesterday from a subscriber who didn't change their plan after receiving the email and instead chose to cancel, but the cancellation was too late and their plan renewed for one last month first. That month was still charged at beta prices, so I suspect the email is supposed to be informing us that we're about to get our last renewal at beta prices and we're getting the email only once it should be too late for us to make changes. So you will have to wait and see, but if you didn't try to make any changes to your plan in the last month I think you'll see one more charge at beta rates tomorrow and then next month you'll get introductory period prices, but you'll have a month to decide on how or if you want to change your plan first. This is just speculation based on my observations, of course.