Submitting force_sa shouldn't be needed. In some cases, where features incompatible with the new layout are used (Instant Update API), there will be fallback to the legacy layout.
Would you be able to post your API call? I just tested the EC flow in both Sandbox & live and I'm getting the new layout.
Try mine:
https://api-3t.sandbox.paypal.com/nvp?USER=pro22_1222868134_biz_api1.paypal.com&PWD=XPBWBJ9AFZ9X748L&SIGNATURE=ABhqmE8JdSxFSHp2C6TGP6yXBWsJANNpoeLALseoabPIe162v-muDlb0&VERSION=109.0&METHOD=SetExpressCheckout&RETURNURL=https://stackoverflow.com/questions/44453010/old-paypal-checkout-layout-on-express-checkout&CANCELURL=https://stackoverflow.com/questions/44453010/old-paypal-checkout-layout-on-express-checkout&AMT=0.01&PAYMENTACTION=Sale
Take the token from the response, change the %2d in the token to a - and redirect yourself to checkout:
https://www.sandbox.paypal.com/cgi-bin/webscr?cmd=_express-checkout&token=EC-47P743568V119643D