Mixing Linq queries and Linq lambdas
There is a reason why you can write LINQ queries using the query syntax or the method syntax: sometimes one is better and sometimes it's the other one.

So, when you're writing a new query, use the syntax that suits your query better. This usually means method syntax for simpler queries (especially if they're as simple as your example) and query syntax for more complicated queries (especially ones with multiple sources).

When you need to use an operator that's only available in method syntax (like SingleOrDefault()), I think that points towards using method syntax for the whole query. But using both syntaxes in the same query can sometimes make sense too.

When it comes to rewriting old code: if it works and it's reasonably readable, I would leave it be, I think the effort spent rewriting it isn't worth it.