It's not as huge as all that, most of Europe is already similarly synchronized and Ukrenergo was already running at 50 Hz. The longer term concern is communication and coordination between the RTOs in Europe and Ukraine, as Ukraine must now respond to grid conditions and act to maintain as spelled out by complex and detailed operating procedures.
Actually syncing up is more of a case of putting in some high resolution meters at the interconnect substation, or maybe some phase shifting transformers, and adjusting the Ukrainian side's frequency by a few micro-hertz an hour until the phase angles are close enough by setting frequency targets at the largest generators. Something similar happens all the time in wide area grids when generators are brought back on line.
Actually syncing up is more of a case of putting in some high resolution meters at the interconnect substation, or maybe some phase shifting transformers, and adjusting the Ukrainian side's frequency by a few micro-hertz an hour until the phase angles are close enough by setting frequency targets at the largest generators. Something similar happens all the time in wide area grids when generators are brought back on line.