Upgrade query

[#1]

I've just transferred over my phone and broadband to PlusNet with a view to moving to a fibre product shortly. My FTTC cabinet is there and ready, Openreach engineers reckon commissioning is all that is required and should be a couple of weeks  - once this is done i'll be upgrading to fibre.
My query is that i've come over onto the Value product anticipating that fibre will be available sooner rather than later. However, depending on how many weeks it takes to commission the cabinet i'll go over the 10Gb allowance. Therefore, should commisioning not go as planned and in a couple of weeks i'm approaching my useage allowance can I upgrade to the 60Gb product and then upgrade to fibre when that comes along? Bearing in mind these account changes may happen in quick succession and I dont want to delay fibre once it is available?
Do upgrades happen 'quickly' or do they get done on the billing cycle? What issues do I need to be aware of? I'm aware that there will be a lead time for the upgrade to fibre as an engineer appt is required but dont want to go to the 60Gb broadband if it then delays my ability to go to fibre..if you see what i mean.
Cheers

[#2]

Fibre upgrades are a lottery and something Plus Net can't control, it is down to BT Openreach engineer availability.  My in-laws have just signed up for fibre and it was 2 weeks at the earliest before they could be connected.  Their FTTC cabinet has been in for a while now so its not liek there is even a new rush in the area.
I would go for the 60GB option just in case as 1GB over the limit will cost you the same as the 60GB limit.  Depends how restrained you can be with your usage while you wait for fibre.

[#3]

Upgrades happen on your billing date so I would suggest you set a £5 extra cost limit to avoid getting clobbered https://portal.plus.net/data_transfer_watch/

[#4]

So if I upgrade to the 60Gb product it wont be effective until my next billing date? Thats 4 weeks away so am hoping fibre will be available by then - so probably best to set the limit. Wish i'd gone for the 60Gb option now, oh well 
Thx for the responses.

[#5]

fibre typically has at least a  two to three week lead time from available to order to installation so don't hold your breath

[#6]

When did you start your current month.  If you ask nicely they will sometimes alter your package mid month 

[#7]

You have to ask really nicely as the latest information is that it screws up the billing system

[#8]

Literally yesterday - just assumed you could move tarif mid month and be charged pro rata if I need be. Then couldnt find anything to clarify, hence question here. Think i'll raise a ticket and ask nicely , you never know I guess.