Is it possible fit a tyre with a different speed or load index?

Yes.

Summer & all-season tyres: you can fit tyres with a speed index and/or load index that's higher than what is recommended by your manufacturer.

Winter tyres: you can lower the speed index by one letter. For example, if your summer tyre size is 205/55 R16 91V, you can use 205/55 R16 91H winter tyres.