Hello Alalaa,
Welcome to the community!
The likely reason is that the system won't allow you to do the change online because you have a plan from British Colombia. There are often Provincial-specific plans or pricing. Your plan current plan may not be available in Québec. You would be able to keep your old BC plan and phone numer, though you should note that people from Québec calling your phone number might incur long-distance charges. You would also need to verify your plan included Canada-wide minutes to prevent incurring LD charges yourself. In addition, many building buzz-code systems can only dial local phone numbers.
Are you on contract? Whether or not a change would affect your contract would depend on your plan type. If your plan is subsidised, you would need to get a plan within the same subsidy level. However, now that Fido has recently switched from their old subsidy model to a financed Payment Program, I'm not sure whether any of those options would be available anymore. If you did need to switch from subsidised plan to a new plan, you might be required to pay the balance of your device.
If you would still like to change your phone number, you would need to contact customer service. In addition, they can also be contacted via Live Chat, Facebook, or Twitter. Those methods can be accessed via the contact page posted above. There would not be any fee for the change since your move involves a change in local calling area (see here).
Hope this helps 😀
Cheers
Hi alaha.
If you make some changes in your account in the last 30 days its could be the situation.
You can wait next at next billing or try to chat with Fido. To chat with fido u just need to click support at Fido.ca
Have a nice day 
Its been a pleasure to assist you today
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Good morning, you might have to contact fido online at Fido.ca and use live chat or go to Fido on Twitter. Take note that you have to use a credit card that has a Quebec address attached to it. A kiosk would work also, as long as you can provide Quebec mailing address and credit card attached to that mailing address. I think that would likely stop you from changing online with self service. The credit card address is important.
