The difference between refinancing and recasting is a bit challenging to wrap your head around because of the fact that refinancing is such a dominant method. Recasting does not mean new loan. Refinancing is a new loan with a reset time length (new end of mortgage date) in exchange for a lower rate. Recasting is the same loan, meaning same interest rate and same end of mortgage date. The easiest way to conceptualize what happens is go to a online mortgage calculator and put in the details of your current loan. Find your current place in the amortization table 5 years in. After you recast it will still be at that same point 5 years in. To figure out what the difference will be, take your current remaining principal minus anything you plan to put towards it at the time of recasting and put that in the mortgage calculator as if that was your initial mortgage amount. Whatever the amortizations schedule now says 5 years in is where you are. It's like going back in time and just changing what you initially borrowed to a lower amount. The ratio of interest to principal will be the same because it is the same loan at the same time point.
So for example, let's say you currently have $750,000 left on the principal and that your initial principal was a million and you recast right now with $50,000. Your new calculated mortgage will be based on the 700,000 not the million. So whatever the table says you owe in combined principal and interest is your new monthly payment excluding taxes, etc. I don't know your specifics, obviously in terms of down payment etc. But I did the math and it looks like at least $2,000 to $2,500 lower payment based on my fake numbers, so with only 500k left you can expect even more. With a interest rate as low as yours, it probably doesn't really matter, but to be clear, recasting makes the most sense. If you're planning on investing the savings and not spending it. Otherwise, you're better off putting that $50,000 elsewhere. In your situation, if your fixed expenses are higher then it makes sense as well to do this because it will make your life easier for several years. You are correct. Since you have put money towards the principal it will reset you to only being 5 years in, but the reality is you are still paying the interest to principal ratio of someone who never put any money towards their principal. It just so happens that by the time you are paying at the 10-year mark ratio you will also be paid off. I think the time that it makes the most financial sense is if you serial recast. This way you are locking consecutive savings and as long as you put that additional money towards the next recast or investments you get compounding benefits because all reductions in principal are factored into the recast, including standard monthly payments. A single recast unless you need the cash flow is not substantially better or worse than just paying off the principal faster. You eventually reach a point of diminishing returns with serial recasting where it just makes sense to take all of your saved monthly payment money and just pay it off. Recasting $100,000 loan that was initially a million for example, probably doesn't make a lot of sense because you have already unlocked most of the monthly payment except taxes and insurance. At that point I would just pay it off.