We don't make any changes to welcome bonus points unless you've switched within the first 90 days of your account opening.
If you switch from Reward to Reward+ during this time you'll be eligible to earn the higher welcome points bonus, as though you opened the Reward+ card.
If you switch from Reward+ to Reward during this time, you'll be eligible to earn the lower welcome points bonus, as though you opened the Reward card. If you have already earned the higher welcome bonus points with the Reward+, you'll be able to keep them.
You need to have made a purchase with your card within the first 90 days of your account opening to be entitled to any welcome points bonus.
Switching cards does not allow you to earn two sets of bonus points.