Answer by Thomas Zerbarini:
The scenario you are asking about is called "displacement." The current aircraft you are flying is being retired so all pilots assigned to that aircraft are displaced to other equipment, seats and/or bases that their seniority can hold.
Displacements work in inverse seniority order unless a senior pilot voluntarily chooses to be displaced first.
What will happen is a trickle down effect as pilots move around to different aircraft. Other pilots will be negatively affected now that there is a reduction in equipment unless the airline is replacing the retired aircraft with other aircraft. Either way the displaced crews to different aircraft will have to go through training to be qualified and typed on the different equipment.
New type training typically takes two to three months to complete. Here is a link to that process previously answered.