Hot solder dip vs Tin-lead plate finish on Microcircuit

Did you ask your supplier what the difference is between the current and proposed finish? You might be surprised, they probably know what they are talking about. However, be sure to ask them specifically what potential problems are inherent to the new process.

Lead-Tin Plating and Solder Dip are basically the same coating. Dipping will result in a thicker coating because of the surface tension. The melting temperature is determined by the ratio of lead to tin.

Can you use an inert atmosphere for your reflow process?

When I worked on a hybrid manufacturing facility, not long out of university, most of the high temperature reflow work was in an inert (nitrogen) atmosphere, and the really awkward stuff was done in a reducing (hydrogen) atmosphere without flux. The latter needs special equipment and, yes, it is as expensive as you might think.