Problem: Customer needed a way, via PowerShell to convert a column from single line of text to Multi-line of text. I first verified that the column they were trying to convert was not the default ‘Title’ column, I would not recommend touching that one. The customer had run into an issue where the Single Line of text column’s 255 character limit was truncating some entries.
$list = $web.Lists["NameOfList"]
$column = List.Fields["ColumnName"]
$script:noteFieldType = [Microsoft.SharePoint.SPFieldType]::Note
$column.Type = $noteFieldType
NOTE: Remember to run these scripts with the correct permissions. When the customer ran it the first time, they had some vague errors like 'Cannot index into a null array'. Found out they were logged into the server as themselves, once they logged in as a farm account, the script worked fine.
Fight Comparison: Mike Tyson Vs. Carl ‘The Truth’ Williams…easy KO
I have found that just changing the $column.type doesn't quite do enough when changing from a single line to a note type. Some of the parameters still looked like they were coming from a text field instead of a note field. The trick turned out to be also
entering the new $column.SchemaXml information, then the full field updated the type correctly