Should bottle conditioned beers be consumed quickly after purchase and do they carry on changing taste for a while? Just wondering how a bottle conditioned beer will change over its life - if at all? And is there a perfect time to drink one? Also do different types change differently?
Quite the opposite, bottle conditioned beer is perfect for aging. The yeast carbonates the beer and will produce slight changes over time, like upping the ABV. This needs to be done in climate controlled environments similar to wine. The yeast will settle to bottom and you want to avoid pouring this into your glass when finished. 

I brewed a Imperial honey stout and aged a few bottles at 68f for about a year for my birthday. The beer started at about 7.2 ABV The beer finished at over 9 ABV (this is because the yeast had plenty of starch to munch on and the carbonation was not affected.