It's partly semantics and partly implementation strategy.
The ACA isn't a single provision, it's a maze of interlocking pieces that has hooks in every aspect of life in the US. It can't simply be terminated or replaced overnight because it would be like trying to unscramble an egg. When all is said and done, there will be a system in place with differences from the ACA. As the ACA, itself, demonstrated, trying to implement change of this scope and complexity in a single step leads to unanticipated problems.
What does replacement really mean?
Any legislation dealing with the subject will have massive overlap with the ACA, and the objective has always been to retain certain beneficial features, like providing for pre-existing conditions and allowing young people to stay on their parents' plan. So even a complete replacement will have certain similarities.
No change can be implemented overnight because of the lead times required to implement them. No matter how change is accomplished, the result will be a blur of old and new that will stretch over a period of more than a year.
In the meantime, changes need to be made because the ACA is crumbling under it's own weight. Some things in the current plan need "repair" to survive until more permanent structural changes are in effect.
Beyond that, it is simply good business practice to implement change incrementally to the extent practical. Some structural elements have dependencies that require concurrent implementation. Other pieces that have turned out to be problematic can be individually fixed, at least temporarily. There is also a balancing act of implementing changes in a deliberative, and to the extent possible, incremental manner, vs. having a stable and predictable environment for all of the players.
Getting to the finish line will be a combination of immediate patches and fundamental changes. The election changed the landscape for what can be changed and how that change can be accomplished. Given that, efforts have been refocused to reflect that new reality. That includes both the "what" and the "how".
The objective for the end result is fundamentally unchanged, although the political realities may allow minor differences from what was contemplated during the Obama administration. Now that Republicans are in a position to actually implement substantive change, one of the first steps is to figure out the best way to accomplish that. Strategy details have moved from campaign rhetoric to implementation plans, so refinement is to be expected.
There is also an element of encouraging bi-partisan participation, so terminology like "repair" is more palatable to Democrats. And I suspect that current calls for "repeal" by some Republicans is part of the posturing and negotiating dance.