I just booked a BE fare as a test on a wide-open flight and do not see any FCM seat upgrade offers for C+ or FC seats. The only offer I see is $29 for Preferred seating (I believe these are variable based on stage length). While FCM offers from Main to C+ or FC can certainly be less than fare diff, I don't believe you will ever see any FCM offers on a BE booking. If OP is happy with a Preferred seat and doesn't care about C+ seats, then maybe that's okay.

BE domestic fares are supposed to have a $99 fee to cancel if you want to rebook it as a Main fare (after 24 hour risk-free period where it is free). While an agent might waive it, it's not something I would advise taking a risk on if you think you might want a Main fare or make a change to flight. As noted above, fare diff from BE to Main can also be substantially more if you wait until closer in due to advance purchase requirements on cheaper fares.