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.

I think you're likely to regret this change. Your bike is a classic and a newer one will be little different in my opinion, definitely not faster. Spend your money on refurbing your bike if you think that's necessary. How about some pictures?

A good condition, mid 80s King of Mercia with top end Campagnolo should make £600+. You mention you've replaced some of the original Campagnolo but not what you've fitted instead. That may affect the price slightly. Some pics would be good

I assume you currently have downtube shifters and the 2004 Mercian will have STI shifters? That will probably be the main noticeable riding difference, but I'm not sure you're going to gain very much else. Is it possible for you to try the 2004 for a few days?