I am afraid, your requirement isn't much clear, however, what I understand from your snippet is that you are reading the condition rule applied on a cell and trying to replicate it for another condition. If I am correct in my understanding then you can use the Cell.GetValidation method to retrieve the validation rules for a particular cell, which in turn contains the Formula1 & Formula2 properties. Please check the following piece of code for better understanding.
var book = new Workbook(dir + file);
var sheet = book.Worksheets[0];
var cell = sheet.Cells["A1"];
var validation = cell.GetValidation();
int index = sheet.ConditionalFormattings.Add();
var collection = sheet.ConditionalFormattings[index];
index = collection.AddCondition(FormatConditionType.Expression, OperatorType.None, validation.Formula1, validation.Formula2);
That said, in case you still face any difficulty, or my understanding of your presented scenario is not correct then I humbly request you to please share a working copy of your code (using previous release where there are no issues) along with the supporting spreadsheet in Aspose.Cells support forum for thorough investigation.
Note: I work as Developer Evangelist at Aspose.
@NSN, I have amended the code as follow. Please give it a try on your side.
var book = new Workbook(dir + "book1.xlsx");
var sheet = book.Worksheets[0];
var cell = sheet.Cells["A1"];
FormatConditionCollection [] formatConditions = cell.GetFormatConditions();
var formatCondition = formatConditions[0];
int index = sheet.ConditionalFormattings.Add();
var collection = sheet.ConditionalFormattings[index];
index = collection.AddCondition(FormatConditionType.CellValue, OperatorType.Between, formatCondition[0].Formula1, formatCondition[0].Formula2);
collection.AddArea(CellArea.CreateCellArea("B1", "B2"));
collection[0].Style.Borders[BorderType.BottomBorder].LineStyle = CellBorderType.Thick;
collection[0].Style.Borders[BorderType.BottomBorder].Color = Color.Red;
book.Save(dir + "output.xlsx");