Replacing a classic Mercian with a newer version

I am considering selling my 1985 King of Mercian tourer and buying a more recent second hand Mercian tourer to replace it (made 2004) - the size and geometry of the newer bike is almost identical to mine. The frame of my bike is in very good condition (one very small dent) and it still has the original handlebars and Campag shifters. However, I have gradually replaced most of the remaining parts over the years (brakes, wheels and front chainset within the last year). I use the bike as my general commuter bike and for light touring - it has served me very well over the last 28 years. However, I feel the newer version (653 tubing, 105 transmission and brakes) will be faster and more suitable for my needs - my primary concern is function rather than classic looks. So, I am seeking advice on:
1. Are there any disadvantages in making such a swap?
2. What sort of price should I pitch my Mercian at?

Thoughts, advice gratefully received.

Hi,

not sure about the advantages, but I'm in the market for a tourer - what size is your 85 Mercian please?!

Actually, just my tuppence worth, but if you've replaced many of the parts on your old machine, not sure how much of an upgrade you're going to experience, beyond the shiny 'newness' and associated placebo effect of having a new toy!? I could be completely wrong about this though...

assuming yours is 531 tubing i don't think you will see much difference between it and 653. And whether you will see a difference between the other stuff depends on what you upgraded yours to.