The rental price can change prior to lease signing but it's usually because of some negotiation between the tenant and LL. Usually the tenant asking for a lower price or the LL adding an optional amenity for an tenant which would cause a slight increase.
You might have some issues with the applicants who have already applied. They came to your apartment because the price was right. To change it now would be lead to them feeling misled and lied to.
The applications you have should be reviewed and I would keep the rental price at what you advertised or quoted if one is approved. None may qualify. Relist the rental at the higher price after the batch of current applications have been dealt with.