The only time I send them in is if I am going to switch leasing companies or when it is absolutely necessary. The letter of intent triggers all sorts of monkey business at the leasing companies.....all of which the leasing companies will deny ever happens....even though I've caught them red handed several times. You have to figure out if your leasing company will give you "Penny Quotes". Basically there is a tiny window between when the last payment posts and when the next payment is invoiced where if you hit it just right the upgrade to return will be $0.01
Also, do not do business with any leasing company that does 12 month renewals. All of my leasing partners are one month renewals or 60-day to the customer but one month to us on upgrades.
As far as end of term goes we have a spreadsheet of all lease expirations and I put an appointment 3, 4 or 5 years out in my outlook for 90 days before a lease ends. I set that appointment as part of my process when I get the new lease signed.