DYI upgrading B&W N805 to Diamond 805?

Parts quality matter.  This is so true in speaker crossovers and audio electronics. Many well respected manufacturers offer upgraded versions on their gear with Vcaps, silver wire, Duelund caps, fast recovery diodes, Output transformers, shunt stepped attenuators, tubes and on and on ..... They also know parts make a difference.  

Speaker builders selling through distributors have to manage build costs and therefore place $ .20 cent resistors and $5 - $15 caps in their speakers. It is done for cost reasons.  

It is a lot safer to upgrade the caps for modern speakers than for speakers from the 80's.

You don't run into ESR differences luke you did before, which does funky things to the impedance curve.

I am now debating whether to attempt this at all, to me, just upgrading caps at random without thinking of what the design and measurements at B&W WITH original Caps were meant to achieve, is not wise. I may make things worse.

Now, how do I find EXACT SAME spec caps, which wouldn't really change the design parameters and do an upgrade like that?

By the exact same spec we mean same resistor ohm and wattage values and capacitor uf values. These are printed on the parts and can also be measured with a simple multimeter. You are replacing film caps so no real ESR issues to obsess over.