Changing from trifexis to Nexgard Spectra or change from Simparica Twill it harm your dog

Our outside Dog, Lilly is a Red Chow/Blue Healer mix and she's been on Trifexis for years. We need a Pill that protects against Ticks also. I did research and Nexgard Spectra appears to be best choice. We get the Trifexis from our Vet and it's gone way up in price to $150 for 6 months.

We also have a small house dog, Wickie, he is a Che/Shee mix and is on Simparica TRIO and it's gone up in price to $156 for 6 months.

I need advice to change them both to Nexgard Spectra if it will hurt to change their medicine. We had heard you can never change a dog off Trifexis it's dangerous? Please help