Evans left BCD M&I earlier this month and has joined Wyboston Lakes as commercial director after eight years at the agency.
Crewe, who joined BCD M&I from Chambers Travel Management in May 2012, left her role in August.
Evans told C&IT: "There is a lot of significant business going on at BCD M&I right now and with this level of change, of course it causes unrest. However, it is one of the top agencies and I believe it will remain to be.
"Senior peers at companies including CWT and AMEX have all done similar things. RFP activity over the last few years has been intense and now the market is on the up there are opportunities to do different things. People are ready to find the next chapter."
Sue Burgess, senior director of operations, BCD M&I, said: "We are going through the recruitment process for Karen’s replacement and a new appointment will be announced shortly.
"The business is continuing to grow, with 23 new staff joining the agency this year, and Jeroen Duiker has been appointed VP SMM Account Management for Europe as we continue our strategy to expand further into European markets."
Duiker has been at BCD M&I since 2011, having also joined from CWT, and has held roles including regional account manager EMEA and director mainland Europe.
BCD M&I also appointed Ben Hoeksma, former programme manager at CWT Meetings & Events as account development director in July.