There’s a tower upgrade (you get by buying levels of your tower and then buying the upgrade) that lets you change them for ounces. It’s not really worth it because pretty soon you’ll have tons of lands and have unlocked the entire lab. Don’t be picky on land conquering – it becomes mainly for money. Choose isolated lands as those take less portal energy to take.
EDIT: Some other tips: Don’t go for forest – it’s not very good. What I did was bought 2 regular pro (ounces only) packs on sale and went with the best cards I got (Dirr, a Promo mountain). I then built on that. Seeing as there’s a new limited set coming, you might want to avoid buying a ton of standard (imperial) packs because money will soon, soon run out if you just started – and then you won’t be able to afford the limited thing.