My 1st fwc (full water culture) orchid was a rescue phal, zero "traditional" roots, only about 5 air roots. I stuffed them into a Mason jar, and filled the water to consume the bottom 10%ish of the roots, it converted easily. I now have all 4 of my pals, and all of my oncidium "types" (multiple oncidiums, 2 brassias, 2 brassidiums), and my lone vanda in full water culture. The phals varied in there conversions, only 1 had the original roots completely die off, the other 3 just made the change. The oncidiums and vanda took to it immediately, though I will say it was maybe less than 10% of the roots were in the water full time at the beginning. What works for me may not for everyone, but FWIW, here is my schedule for every single one of my orchids in FWC:
All except the vanda and 2 brassias (has been outside since spring, will come in before temps get to low), are grown indoors all year round. Bottom 10% of the roots are in water full time. Every sunday, the jars are filled right up to, but not including the crown, completely submerging the roots for about 1 hour, with k-lite fertilizer at about 25ppm N. Once a month, a long with the k-lite is a tablespoon each per gallon of Kelpmax and Inocucor. After the fertilizer soak, the water is changed out for fresh new water (this is very important as I have just found out recently). This has worked for me for around 2 years of FWC.
Just to see what would happen, I stuck to the regimen for 2 of my phals, the other 2 I treated slightly different, I did not change out for fresh water. You will clearly see the roots starting to rot on the 2 that didn't get the fresh water every week. All 4 have spent just as much time, at the same depth in water, but clearly changing it out is key. Btw, the flowers on the left, be long to the plant all the way to the right