sorry i'm late to the party...
but i agree with everyone... it's easier to work around the center stone... so finding the center stone is most important... usually custom settings are made for the center stone... it'll be harder later on to replace the center stone than reset the center stone in a different setting...
and as far as upgrading goes... i don't know how many people out there would actually upgrade their original ER... it just has too much sentimental values to be upgraded... however, what women do like is getting more rings! RHR or alternate ER for example...

so to make my point, get it right the first time!

Thanks Alex for telling it from a male perspective from someone who got married not too long ago.
The center diamond is the CENTER of attention. I want a pink lemonade ring and I could kick myself for not buying the light yellow princess with light pink side stones 4 years ago. I thought about it ONE day too long.