I agree with
@gottalovepluto . I don't think you will get very far with Target Corporate. They will contend that Target sells a wide variety of merchandise and your GCs have value. I highly doubt that you will get your money back because you banked your vacation money in Target gift cards instead of a safer place. Your options are either to use them for items for which you have a need or sell them (at a loss) thru one of the gift card resale websites.
Last month, I (and a few others) got "stuck" with BB GCs when they changed their GC-> GC policy without warning. I found a way to make myself whole (replaced a cellphone that Samsung could not repair under warranty and they prorated a refund to put towards a new phone). I also got caught with $75 in Target GCs that could not be converted last week. I bought some toilet paper, coffee and groceries. I still have $16+ to spend down, And who could forget the great deal of weeping when eBay unceremoniously eliminated the ability to purchase a GC with an eBay GC? That effectlively ended the fun we all had churning from eBay GC <-> Target GC and finally to Disney GCs. Yep, got "stuck" with a few hundred in eBay GCs that I found a way out of.
I hope you can find a way to may lemonade from these lemons. It's a great deal of money to have tied up and to be unable to use it as intended. It's been said in this thread many times - don't risk more than you're willing to lose when it comes to playing this game. The rewards can be great but if the rug gets pulled out from under you - UGH!!!!!