Can my membership tier or welcome bonus go down if my balance drops?

Your membership tier is based on the greater of your current balance or your lifetime funded amount (approved deposits plus any positive manual credits). Because the lifetime-funded figure only ever goes up, your tier is effectively anchored by the total you've put in — so trading losses or a withdrawal that lowers your balance won't drop you below the tier your deposits have already earned. A welcome bonus that was already credited to your Credit stays credited (each tier's bonus is granted once). Note that bonus funds in Credit are not withdrawable and never count toward the deposit total, so you can't use a bonus to reach a higher tier.