@David J. has some good points ...
If I were in your shoes ... I'd have a sit down talk as well. Go over the comps in the current rental area, and explain that you love her as a tenant and wish to see her stay on board, but you must raise the rent to more marketable rates. See what she is "ok" with doing. Whether that's $50 every 3 months, or 6 months, etc... etc... I'd have a new lease spelled out with the new terms drawn up ~ go over them with her, and go from there. If she balks and wants to move out, let her. If she agrees, look at the bright side ~ you've got a long term tenant who has proven she'd like to remain such.
If a long term tenant (18+ years) had 18 year old carpet, never complained about it, and paid rent on time. I could care less about redoing the carpet or paint. Whether she moves out tomorrow or 5 years from now, those items will still need to be done. So whether or not she continues to reside there doesn't make much of a difference.
I could care less about the tenant being a hoarder. And I feel a $150 increase (nearly 25% increase) demanded by the Landlord may fall upon deaf years. See if you can reach common ground and raise it slowly :)
My $.02 cents! Good luck and let us know the outcome!