Replacing fuse

Hmmm. So you want to buy a fuse on sale, save a few bucks and as a result burn your house down ... makes a whole lot of sense! 

Don't put in the wrong fuse -- the warranty is immediately void, and you create an electrical risk

It will work ... but the fuse isn't doing the job it's intended

6.3A is a standard size and all the audiophile fuse manufacturers make this size -- Synergistic Research red/black/blue for example all available in that size
https://highend-electronics.com/products/synergistic-research-blue-quantum-fuses?variant=44898590531

I guess my concern is that the OP is clearly a neophyte to fuses and uncomfortable with the whole process, so the safest advice is to stick with the rated fuse

My own experience with SR fuses is that they do tend to blow on start up so I as a rule go "one size up" which would be the 8A for 6.3A, but I do this eyes open and well aware of the risks I am taking.

I think to recommend this for someone who is making a decision based on stock on hand and saving a few $ is the wrong advice. Most likely you'll not have any issues agreed but why risk it?

There are of course those who swear by bypassing the fuses completely ...