The set of practical stills is quite different. I would say that it is basically impossible to work with modern experiments if you haven't done an experimental Ph.D. The converse is a bit easier, as anyone with a Ph.D. in physics can play around with equations.
At what level are you currently studying? From your post history, I get the feeling that it is a bit early for you to ask yourself such a question.
The transition from experiment to theory is easier than from theory to experiment. The thing to worry about is how you are going to earn a living during the transition. Transitioning to the point where you can have reliable grant funding and stay on tenure track at an R1 school is a bigger challenge than just doing productive work in the field and publishing papers.
If I had a PhD in theory and wanted to work in experiment, I would get a teaching job at a teaching focused school and offer my services for free to an experimental group at a nearby R1 university. Odds are you can find one that will have you, and if you show up regularly and are hard working and careful, the grad students will begin showing you the ropes in their experimental operations. If you are making real contributions, you will eventually be offered co-authorship on some of their papers. This is probably the most dependable path to productive experimental work.
There are more options for someone with a PhD in experiment who wants to work in theory. Since theory is much less expensive to pursue, you can just start working on it with whatever skills you have if you find a problem that matches those skills. You can try and attach yourself to a local theory group at a nearby university, but you need to consider what skills you might have that will interest them. Good programming skills are likely a ticket if you are willing to work for free.
But there are not many paths for a theorist to get paid to become an experimentalist and vice versa. You gotta do it for love rather than for money.
I've known theorists who have joined experimentalist groups (to work as theorists) who have gotten increasingly involved in experiments, almost by exposure. They're still predominantly theorists, but do contribute to experimental work. But that sort of thing relies on being in the right sort of group, which is not necessarily something to rely on. But is it possible? Yes.
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