I have changed curriculum providers, but I think I've done well keeping my dd10 on the curriculum continuum. Yes, I've changed from Ann Ward's preschool to Sonlight for K and first grade to Heart of Dakota for 2nd through 5th. However, they are merely tools. Yes, if I decide a certain book or style is not for us, I switch. My dd is a natural speller, and ironically that means it is hard to fit her into a spelling program. I tried lists, then Spelling Power, then Spelling Wisdom and HOD dictation before finding Christian Liberty Press Building Spelling Skills. Now, in 5th grade I feel both HOD and CLP BSP will serve us well for years to come.
If it's not working, ditch it quick! Try to recoup the money by selling it online. And count your lucky stars that you aren't beating your head against the wall with it. LOL.
That said, I haven't changed all that much. I'm one of those agonizers. I ask for opinions, research, ask more opinions, look at reviews, etc before I buy anything. My biggest change was from k12 to traditional in the middle of a year (and from ps to k12 in the middle of the previous year too), so obviosly Im not afraid to ditch it when it stinks!
We changed as often as we needed to. I wouldn't pay out more money though. We had to get creative with free resources instead. It helped that we never bought a "full curriculum" and only pieces of curriculum.
Also POC4U helped us fit the curriculum to the students much better.
We switched up US History last year. We went from text-heavy "A Patriot's History of the US" to using more video and internet resources like "The Story of US" from PBS. In the end, I feel as though she got a solid overview of our history, plus a ton of discussion and real-life examples.
But I stuck with the text for about 4-5 chapters, even though she was miserable by the end of Ch 1. Lesson learned. We would have been done with history and had much less angst if I'd switched as soon as I realized it wasn't working.
Our first year, we struggled through. Our second year, we dropped ALL of it, and used a completely different curriculum. This is our third year and we liked our last years curric we are using the same brand and only slightly supplementing for fun with another.
I switch curriculum a lot. There were things I thought my kids would love and they hated, and things I wasn't so sure about and they just 'got it'. I rarely invest in expensive or new anyways, so it isn't that big of a deal for me to switch out a book. I buy used as much as possible, and use internet and my printer a LOT for supplements, as well.