No water bath jam

Hello,

I made my first batches of strawberry jam in the fall. I still have some jars left. The person who showed me what to do told me that filling the sterilized jars, sealing them, and turning them upside down until they were cool would be enough to create a good seal. So I made them that way.

Since then, I haven't heard of anyone else doing this and now I'm afraid that they aren't safe to eat. I've already been through several jars just fine but I'm worried about the others. Is it possible to boil the jars now without opening up the jars, reboiling the jam, and putting them into newly sterilized jars? I made them with about half the sugar since the strawberries were so sweet and the juice of about half to a full lemon per batch.

Are they fine? Should I store them in the fridge? Please help!

For future reference, if I use less sugar, what would the boiling time be for the filled jars?

Thanks!