Excited states generated in energy upconversion process in NaYF4:Yb3+,Tm3+ nanoparticles may enable a range of biomedical and material applications. We describe preparative and post-preparative control of upconversion, with particular emphasis on ultraviolet upconversion. A series of hydrothermal syntheses were performed with ytterbium doping ratios ranging from 0 to 99.5%. Initially, UV luminescence intensity increases substantially with an increased presence of ytterbium as ytterbium participates in energy transfer upconversion to improve luminescent yield. However, as the doping ratio of ytterbium increases, substantial loss of luminescence occurs due to energy migration through ytterbium, likely through surface sites, non-radiatively dissipating energy from the system. We investigated NIR and UV luminescence from these UNPs under conditions of pulsed 980 nm excitation, with the pulse width varied from 10 to 2000 µs. The duty cycle was kept constant so the average power (energy/time) delivered by excitation was the same for any of the pulse widths tested. We observed that the UV and NIR emission intensities have vastly different responses to changes in excitation pulse width in this range. NIR emission intensity changes relatively little with the pulse width. In stark juxtaposition, UV luminescence shows three orders of magnitude reduction of intensity upon changing excitation regimen from a long pulse width to a short one. The highly nonlinear luminescence response to pulse width enables control of UV luminescence intensity in the manner resembling that of a binary switch. Through the use of preparative techniques and a novel excitation modulation scheme, we demonstrated control of upconversion pathways, so NIR to UV upconversion can be turned on and off while maintaining NIR to NIR upconversion, in support of applications combining photochemistry and imaging.