I recently have gone through the effort of understanding this, so hopefully this can help:

There is a $75 fee to redeposit the miles. I'd discourage one to do that in a situation where an award has been claimed.

Better option:
One can change the award ticket flight to another award ticket flight without a $75 fee, if the flight that was originally booked is not within 8 days. The new flight can be within 8 days also. It's just the original one can't be within 7-8 days.

It can be different city pairs/route also. So, if I have a flight from TTN-TPA in two weeks that I know that I can't make, I'd just call the agent, and request that I want to convert that award to IAD-MCO for two days from now, assuming the award is available, and not have any fee to pay for it.

However, if it's today and I have a Standard Award ticket flight just 5 days from now, if I change it, there's a $75 fee.

However, I can also request the agent to cancel the flight and put the award on hold. I then have a year from the purchase date to use the award and can pay the $75 fee when I actually re-book it. I have one award reservation on hold in such manner.

It's obviously not as good as Southwest's no change fee, but it can be viewed as better than Delta's $150 award change fee where any type of change is $150. Of course, it's not apples/oranges, and Delta has a huge network and connection choices. Relative to Southwest, it's easier for me to find a 10,000 PHL/TTN-Florida ticket than it is to find a 10,000 point Southwest ticket out of PHL.

But the mileage program on Frontier is obviously becoming a weak point as Frontier's network dwindles.

I wonder if they will discontinue this FF program altogether to cut costs as there is an overhead of maintaining this FF program. Instead, it could just offer frequent fliers that fly maybe 20 segments, coupon codes for discounts that can be used on top of other codes maybe for even more discounted fares. I also think Classic Plus might be not necessary in the ULCC model, and an overhead to maintain two fare levels, but maybe it could change the change fee from a flat $75 to 20% off the fare, pleasing some pax that might otherwise not like the all the other changes.