Convert Flexform XML string from database to array, convert array to Flexform XML string to write to database in TYPO3 - typo3

Question

Question: How to convert Flexform XML to a data format where settings can be migrated and then write it back to DB. Are there some pitfalls or things I have to consider? Is there any reason not to just use GeneralUtility::xml2array() and GeneralUtility::array2Xml?
TYPO3 v11 / v12.
More details:
This is a common use case for me in my extensions: Often the schema of the Flexform is changed to improve usability. For the sake of the editors I want to migrate existing values to the new fields.
This would be pretty straightforward (in an Upgrade Wizard or Command):
Get the XML string representation from $row['pi_flexform']
Convert it to an array
Make the changes in the array
Convert it again to an XML string representation
If this is different from previous, write to $row['pi_flexform'].
The problem is figuring out which TYPO3 functions to use. There is FlexFormService, FlexFormTools and there is GeneralUtility::xml2array() and GeneralUtility::array2Xml.
The simplest solution seems to be to use array2xml and xml2array.
The second problem is figuring out what is valid XML for Flexform (unless the functions already take care of that).
Let's say I had something like this in pi_flexform which should be converted:
<?xml version="1.0" encoding="utf-8" standalone="yes" ?>
<T3FlexForms>
<data>
<sheet index="sDEF">
<language index="lDEF">
<field index="settings.foo">
<value index="vDEF">content of foo</value>
</field>
...
I don't really need the language index but that is how it looks now.
The foo field is gone in the new Flexform and the content should be converted to:
....
<field index="settings.bar">
<value index="vDEF">content of foo</value>
</field>
...
The real use case is more complicated and actually makes sense ;-)
Unfortunately, the API is not so clear.
use TYPO3\CMS\Core\Configuration\FlexForm\FlexFormTools;
use TYPO3\CMS\Core\Service\FlexFormService;
Convert XML to array: Solution 1
$xml = $this->flexformTools->cleanFlexFormXML('tt_content', 'pi_flexform', $row);
$xmlArray = $this->flexformTools->cleanFlexFormXML;
Result: If the Flexform schema only contains the new fields, the old fields are removed - so we can't use this if the schema changes
Convert XML to array: Solution 2
$xml = $row['pi_flexform'];
$this->flexformService->convertFlexFormContentToArray($xml);
Result: This works, but the resulting $xmlArray looks like this ['settings' => ['foo' => ...
can be used for flexforms with settings but probably not best choice if is to be written again
Convert XML to array:
solution 3
$xml = $row['pi_flexform'];
$xmlArray = \TYPO3\CMS\Core\Utility\GeneralUtility::xml2array($xml)
Unclear: what should be in second and third parameter?
the result actually looks good: ['data' => ['sDEF' => ['lDef' => ['settings.foo]
Write array to XML: Solution A
$xml = $this->flexformTools->flexArray2Xml($xmlArrray, true);
Write array to XML: Solution B
$xml = GeneralUtility::array2xml($xmlArrray);
Solution A would write it like this (if combined with convertFlexFormContentToArray). Apart from that they should both work.
<?xml version="1.0" encoding="utf-8" standalone="yes" ?>
<T3FlexForms>
<settings>
<link>http://csd.informatik.uni-oldenburg.de/adam/adamMC.bib</link>
<sort>none</sort>
<sortfixed>0</sortfixed>
<filterType>none</filterType>
<filterEntries></filterEntries>
</settings>
</T3FlexForms>