Generally Yahoo queries account for about 24% of normal daily click-throughs (compare to Google's 40%) - when Yahoo makes the switch - the likelihood that any significant change to that number will occur (up or down) is very small.
Notwithstanding... will Yahoo regain or surpass their previous levels of market share?
Anything is possible - but an instantaneous jump from Google to Yahoo "on switch" will never occur...
This tends to be a concern only to SEOs/web site owners that know/do
SEO as for the general markets (searchers) really don't care about technology, rankings, algorithms... if they continue to find what they want and (probably more important) like the interface of the search engine they are at/features/look/style/ etc. they will not move.
My wife - refuses to use Google - because she started with MSN - and Google isn't organized as MSN - thus she is happy with
"what she knows and understands"... aren't we all? That's her comfort zone - and no algorithm or company brand change (search change) will change her - until MSN makes a mistake.
We are all creatures of habit.