The content is already stored in the repository, so the only way you can fix this is by writing Groovy Script, the below script can help you to fix this
Note: the code is not tested and it is self-understanding you can modify it if it does not work for you
def search = "<b>"
def replace = "<strong>"
def path = "/content"
def property = 'jcr:description';
def query = createSQL2Query(path, search , property)
def result = query.execute()
result.nodes.each{node ->
def description = node.get(property)
println description
node.set('jcr:description', description.replaceAll(search ,replace))
println node.path
}
save()
def createSQL2Query(path, search, property) {
def queryManager = session.workspace.queryManager
def statement = "SELECT * FROM [nt:base] AS s WHERE ISDESCENDANTNODE([${path}]) and s.[${property}] like '%${search}%'"
def query = queryManager.createQuery(statement, "JCR-SQL2")
query
}