Error while upserting PricebookEntry

Question

Issue I am getting this error while upserting pricebookentry on an external ID

Batch_RDM_PriceBookEntries_Update:execute:errorMessage : The following error has occurred.INVALID_FIELD_FOR_INSERT_UPDATE: Unable to create/update fields: CurrencyIsoCode

1. We want to create the pricebookentries when they are not present and update them when they are available
2. Inserting is working fine, however, while updating I get this error
3. I am aware that I cannot modify Currency, Product and Pricebook Ids however, I am not trying to do that, I just want to update the cost and price when we update.
4. We cannot delete the existing pricebookentry and create a new one, as the pricebookentries are used in other objects and deleting pricebookentries will break multiple things
   
   What am I doing wrong, any help would be appreciated

Code

for(RDM_Price_Book_Entry__c RDMPbe:scope) {
            PricebookEntry pbe = new PricebookEntry();
            
            pbe.Product2 = new Product2(RDM_Id__c = RDMPbe.Product_External_Id__c);
            pbe.Pricebook2Id = RDMPbe.Unit_of_Measure__c=='Days'?PBDailyRate:PBHourlyRate;
            pbe.IsRDMReference__c = true;
            pbe.Cost__c = RDMPbe.Standard_Cost__c;
            pbe.CurrencyIsoCode = RDMPbe.CurrencyIsoCode;
            pbe.RDM_Id__c = RDMPbe.Unit_of_Measure__c=='Days'? //External Id
                            PBDailyRate+RDMPbe.Product_External_Id__c+RDMPbe.Unit_of_Measure__c+RDMPbe.CurrencyIsoCode:
                            PBHourlyRate+RDMPbe.Product_External_Id__c+RDMPbe.Unit_of_Measure__c+RDMPbe.CurrencyIsoCode;
            pbe.Last_Update__c = operationId;
            pbe.IsActive = true;
            pbe.UnitPrice = RDMPbe.Target_Price__c;
            system.debug('Batch_RDM_PriceBookEntries_Update:execute:PricebookEntry RDM ID'+pbe.RDM_Id__c);
            PBEntryToUpsert.add(pbe);
        }
        
        try{
            Database.UpsertResult[] upsertResult = Database.upsert(PBEntryToUpsert, PricebookEntry.RDM_Id__c, False);
                
            for (Database.UpsertResult saveResult : upsertResult) {
                if (!saveResult.isSuccess()) {
                    // Operation failed, so get all errors                
                    for(Database.Error error : saveResult.getErrors()) {
                        errorEncountered = true;
                        errorMessage+= 'The following error has occurred.';
                        errorMessage+= error.getStatusCode() + ': ' + error.getMessage();
                        system.debug('Batch_RDM_PriceBookEntries_Update:execute:errorMessage : '+ errorMessage);
                    }
                }
            }            
        } catch (Exception ex) {
            errorEncountered = true;
            errorMessage = 'Batch_RDM_PriceBookEntries_Update : '+ex+' : ' +ex.getStackTraceString();
            throw new CustomException('## debug Batch_RDM_PriceBookEntries_Update.execute : ' + errorMessage);
        }
    }

Issue