Taltem, a very warm welcome to FT. You actually find yourself in a very fortunate position that you can amend your LHR-AMS flight for a very small amount of money, particularly when measured against the cost of the following F itinerary.
Your assumption that you've made something of a error is correct - in fact, you have just about the highest risk connection possible. If it's £35 to change, ring up now and change to the B2B.
AMS rates as "Excellent" for a B2B - details in
http://www.flyertalk.com/forum/british-airways-executive-club/1457438-back-back-immediate-turnarounds-ba-destinations.html