Hi BP, I just bought my first STR with two of my close friends/partners and I wanted to put a partnership agreement together to protect us in the case of someone wanting to exist or if an unforeseen event comes up.

That being said, maybe you should opt for month-to-month leases with the existing tenants and create a schedule of increases over the next 6 months. 

Most inexpensive installs do a pocket install where the window is set in the existing opening and fastened through the sides into your house framing...the only way to replace windows in brick or stone...then foam and caulk.

@Andy Park given that the tenants are month-to-month I would terminate the existing lease and create a new one.

A real estate is a place where lots of opportunities exist.  

Analyzing other markets should be the norm...your current market won't be offended.90% of the REI time spent each day on REI should be spent analyzing new markets, and reanalyzing existing markets. 

Joshua Tree is over saturated, so now people are moving to bordering towns.

DIY forum online folks suggested jacking/raising up the existing shed and fixing the perimeter with brick or concrete, etc. to mount the shed onto.

When purchasing a leased property, must you always honor the existing lease?

If you're going to rent the properties out and keep the existing mobile homes there, you'll just have to figure out the market rents and expenses for each unit.