The easy way, the industrial way, was to pay people based upon role – job title – tenure etc.
But the world has moved on from that industrial practice now. Now we need to think about pay through the lense of individual contribution (to the culture, to the development of the team, to their value to clients …) and take into account the market.
Some people are underpaid thanks to the legacy thinking and this needs to be addressed before they leave you. Their replacements will be a whole lot more expensive and the upheaval will be significant.
Try to get ahead of this, and your culture – and people – will thank you for it.