2001 Volvo V70 Transmission won't engage

I recently replaced the fluid in my tranny. Not a full flush, but rather a drain and top-off. Now the transmission won't engage accept in 1st gear. When I try to accelerate past that it just revs without picking up any speed. The mechanic who replaced the fluid says I need a whole new system. UGH. But if this problem is based on a seal that had gaps taht were being filled by the old fluid (and random filings etc), doesn't this new problem just speak to a lack of adequate pressure being obtained? Is this alleviated any other way then now putting in $3000?