If one were to be "fully vaccinated" with 2 doses of the Pfizer for COVID19 and then six months later vaccinate with the Moderna version: What is the word or phrase to indicate such a protocol?
The "CombiVacS" trial used the term "heterologous vaccination schedule", and several other papers cited by that one also use "heterologous":
Kardani, K., Bolhassani, A., & Shahbazi, S. (2016). Prime-boost vaccine strategy against viral infections: mechanisms and benefits. Vaccine, 34(4), 413-423.
Logunov, D. Y., Dolzhikova, I. V., Shcheblyakov, D. V., Tukhvatulin, A. I., Zubkova, O. V., Dzharullaeva, A. S., ... & Gam-COVID-Vac Vaccine Trial Group. (2021). Safety and efficacy of an rAd26 and rAd5 vector-based heterologous prime-boost COVID-19 vaccine: an interim analysis of a randomised controlled phase 3 trial in Russia. The Lancet, 397(10275), 671-681.
Shaw, R. H., Stuart, A., Greenland, M., Liu, X., Van-Tam, J. S. N., & Snape, M. D. (2021). Heterologous prime-boost COVID-19 vaccination: initial reactogenicity data. The Lancet, 397(10289), 2043-2046.
Spencer, A. J., McKay, P. F., Belij-Rammerstorfer, S., Ulaszewska, M., Bissett, C. D., Hu, K., ... & Lambe, T. (2021). Heterologous vaccination regimens with self-amplifying RNA and adenoviral COVID vaccines induce robust immune responses in mice. Nature communications, 12(1), 1-8.
You'll find similar terminology of older studies regarding other vaccines but I wouldn't guarantee that other descriptions aren't used, as well, using common English language.
Fragapane, E., Gasparini, R., Schioppa, F., Laghi-Pasini, F., Montomoli, E., & Banzhoff, A. (2010). A heterologous MF59-adjuvanted H5N1 prepandemic influenza booster vaccine induces a robust, cross-reactive immune response in adults and the elderly. Clinical and Vaccine Immunology, 17(11), 1817-1819.
Leroux-Roels, I., Roman, F., Forgus, S., Maes, C., De Boever, F., Dramé, M., ... & Leroux-Roels, G. (2010). Priming with AS03A-adjuvanted H5N1 influenza vaccine improves the kinetics, magnitude and durability of the immune response after a heterologous booster vaccination: an open non-randomised extension of a double-blind randomised primary study. Vaccine, 28(3), 849-857.