I am wanting to get the conversion kit, so wanted to know if it would work well just using the existing miniatures from 2nd edition. Obviously, for hero's I think it will work out okay to sub as there can only ever be 4 and each person will remember their piece. More concerned if there are monsters that may cause issues, such as, are there any large monsters with dimensions not represented in the 2nd edition box? Also if anyone else has played like this is it confusing or tedious trying to remember which proxy represents which monster group?

http://www.fantasyflightgames.com/edge_foros_discusion.asp?efid=256&efcid=1&efidt=689914

The conversion kit is a no brainer if you want some variety to your 2e. If using the existing figures is confusing to your group I believe there are kits online for printing paper proxies. Get the CK either way. It's a great way to spice up the game.