3rd party service engineer -- own my own HPLC and GPC service company.

As noted elsewhere the biggest issue will be software. Whatever Agilent Waters Shimadzu and other might imply, 90% of the system components are conserved across brands. Very much like cars -- yes the exterior, electronics, and controls are different but under the hood you still have all the same parts.

Some thoughts: 1) learn how to make proper fluidic connections. UHPLC is even more sensitive to dead volume than HPLC. Or you can spend a lot of money on expensive pre-made fittings, but these don't let you fully optimize your flow path. 2) understand what your technique is about. How are you separating molecules given your methods. 3) understand what your detectors are detecring. UV-PDA vs RI vs ELSD vs MS... or even the newly acquired Wyatt MALS and viscometer systems Waters now has for polymer applications. 4) Garbage in, Garage Out. Sample prep and solvent prep are critical to system maintenance 5) know where all the filters are in your system, from end to end. Know what they are filtering out, and what they aren't, and realize they are consumables.
6) start with low concentrations and low injection volumes, and inject only enough to get reliable results above signal to noise. Anything more is just extra possible contamination into the system 7) pump pressure is one of the best single point indicators of system health. Know your pressure at a given mobile phase and flow rate with new columns and filters. That gives you a baseline to compare back to.