Does anyone know if this still applies if I upgrade my 100 fibre plan to a faster plan during the 3 months period?
Sometimes you can have success by (politely) asking if the offer can be applied to you as an existing customer.
Especially if you're upgrading your plan at the time.
I've found the Spark live chat very helpful, and if you have no term currently and see a good deal for new customers - have a chat and see what they can do for you.
Might require a new term being applied to your connection but worth it if you're staying with Spark in any case.
But if you're asking if upgrading your plan would take away the offer if you already had it - no it shouldn't. Downgrading might, but paying more per month should be good news for Spark