You have to manage the situation with empathy. Aside from legitimate technical concerns, which you did not mention, from that person point of view, he/she was

You have to manage the situation with empathy. Aside from legitimate technical concerns, which you did not mention, from that person point of view, he/she was likely the expert, the go to person to understand how stuff worked, or could be implemented, and suddenly he/she will be relegated to a maintenance mode or worse be let go. That might not be true, but that is surely how that person feels. So depending on the exact circumstances, you have to reassure him/her by taking concrete actions, like making him/her responsible of a key element of the new architecture, so that this person will feel empowered and useful in the future. To be blunt, even if the situation is better for the company or product roadmap, in the end, most people don't care. The normal reaction of normal people in face of change, is to consider how it will affect THEM first, they don't necessarily take into account the bigger picture. So you cannot convince them with rationale and objective elements how it will improve the life of everybody else.