I want to switch from watermelon to strawberry milkshake what you think?
Well - I do not like either flavor personally

. I would think that those 2 flavors sort of would get along well together. I actually know folks that drink Strawberry watermelon juice with a splash of club soda and even mix vodka in so ..... I think it is doable.
I'd let the first flavor go low in the tank but not too low to burn and then just dump in the second flavor. Since they do play nice together it even gives you some options because you can mix the juices together in different concentrations. You can try higher watermelon, lower strawberry - higher strawberry lower watermelon or even half and half. It gives you a lot of options.
If that is not your thing then just let one flavor go low, add in the other and after a short period, the first flavor should go away/become more muted but not totally.
Also, in my opinion, those flavors tend to be very aromatic so if you go that route you still might be getting the two flavors mixed. I believe that is a 2ml tank so I think if you change flavors out you will never get one pure flavor without a total change of coil and cleaning. If you prefer just one pure flavor, then I would change the coil, rinse it (I think you can gently rinse it out), let it air dry and get all the first smell out and then start again. That would require some time down, though.