Talk to your new solicitor. They will explain to you that it largely depends on the nature of the work involved. If your current solicitor has given undertakings on your behalf, they will need to be released from those. Simillarly, if they are in correspondence with parties on your behalf, it would be inappropriate for a new solicitor to simply "butt" in without resolving formalities.
The norm is that your new solicitor seeks the release of all files and confirms the situation as regards outstanding fees. It is preferable that any outstanding fees are paid to the first solicitor and the files are released with no preconditions before the second solicitor starts doing any work.
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