Probably pretty high. I'm batting 100% for upgrades out of BDL, given the large aircraft flown to ATL, a high % of leisure travellers (who supposedly do not have elite status) and BDL receives a very low % of Boston traffic.
When I've flown out of Windsor Locks in F, there have been empty seats in F on several occasions. If you find four hours before your flight that many F seats are still available, go to the airport ASAP and get on the standby list. With any moderate degree of luck, you should be able to clear you in F all the way though.
And if not, you have a viable excuse to never fly DL again!