Hi,
river.
The price of a diamond is determined on the Four C's: Cut, Colour, Clarity and Carat. The more yellow a diamond is, the lower it's value and the same with it's clarity (whether there are any imperfections within the stone, although they're rarely visible to the naked eye).
With this in mind, I can't see that the stones that are already set in the bracelet can be of a worse quality than the ones you have been quoted as you're being offered more in the way of points (carat). For them to be of the same worth, this would make sense. (For the same value, one of the "C's" has to be compromised.)
If I were you I would take the bracelet to be valued with it's own stones. Try Ernest Jones. They have an independent valuation team that come in on set days of the week. They can also help you out if you do decide to alter the bracelet, with their 'Design Days'. I hope this helps.
http://www.ernestjones.co.uk/webstore/storeloc atorsearchbyregion.do
http://www.ernestjones.co.uk/webstore/designev ents.do