Stated in the title. I'm looking to go from a 5 piece to an 8 piece drum kit. And by better, I mean both sound consistency between the different pieces and economically better.
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This depends strongly on your old set condition.
Matching the sounds of the old and new hardware is always challenging. The most practical problem is the difficulty of memorizing the sound at home(studio) and remembering in the purchase location. This is already difficult when buying new cymbals.
One pragmatic solution is looking for the original hardware already designed for the existing drum parts say Pearl Studio parts for the existing Studio kit. That would increase the chances of handling the color of the sound. But different conditions of different toms makes it difficult to have a consistent tonality across the drumset.
Another difficulty is the renewal of the 8 piece drumset should you decide to change everything. This is similar to buying a modified old car and a new one. The chances are higher to sell the newer one unless the old one has some collector's item value.
Tom/rack connection mismatches etc. are also minor but frustrating problems that might pop up.
I would recommend selling your old set and replace it with a new one.