Would there be much hassle in switching ( i dont want any down-time)?
You will have to decide how important no downtime is to you.
If you are switching from cable to fiber, then it is two completely different systems.
You can just say nothing to your cable provider until the fiber connection is up (and working) and then cancel the cable subscription.
You will have to pay for both until the cable service ends. That could be a month, if that is your notice period.
The other option is to cancel the cable with 1 month notice to end on the date the fiber is operational. If you get it right there will be no overlapping payments.
Though that puts a big risk that you will be without internet for at least a few days (maybe weeks).
It is hard to know exactly when the fiber will start. They have to send people out to run the fiber.
First they will send someone to do a quick survey. Often the surveys are pointless since the survey guy already knows the housing estate and knows if they are an easy or hard one. But, it helps pad out the time to installation.
Maybe you get lucky and it is easy to run the fiber from a nearby point in your estate.
More likely they do a survey and determine that there is some issue (small or big) and that will be a delay relative to their initial (optimistic) estimate.
It depends a lot on the quality of the ducting near your house, and in the estate more generally. They may have to dig a few holes to get down to inconvenient bends/breaks in the ducting.