This will be interesting to see what everyone says! Personally I do reuse mine, usually by mixing with new, plus other stuff, but I have also just used it in the bottoms of big pots with some added fertilizer, to save on new. Why not have three pots this year, one with old that has been fortified, one mixed and one new and see how they go, then you will know for the future!

Yes you can, but its a risk because you don't know if any pathogens have built up in it over time. One deciding factor is whether what you grew in the compost previously had any infestations (vine weevil for instance) or disease, particularly fungal infections such Pansy Downy Mildew, or rusts, black spot, other mildew infections - if they had any of those, best not re-use, spread it over open soil (beds and borders) instead.

Otherwise, yes, liven it up with some fertilizer, forked all the way through, and risk it.

Unless I know that the compost has not held diseased plants or that there are grubs of any sort in it, then I reuse the compost. Our compost is primarily a mix of loam, grit and leafmould so the leafmould is the only ingredient to have disapeared and this is replaced by fresh plus bonemeal fertiliser.

I use some of mine to fill the bottom of larger pots with added fertilizer. if there have been any grubs in the compost it gets scattered on the borders for the birds to pick through.

Yes I also mix new with old especially with the larger size pots...