[SalesForce] Data migration from one org to another

From what i understand we basically need to re-parent the Attachments with Account Id

First migrate all as it is in current Org to the new org.

Then we can write a small snippet piece in execute anonymous to do the re-parenting .

I drafted a simple algorithm and you may need to tweak a little to get it working based on realtionship you have

1)Construct a map of Contract Id and Account Id(Map)(Query Contract ) 2)Query all the attachments which you want to reparent with parent Id 3)From the parent Id (i.e Contract ) also using the map of Contract Id and Account Id you can obtain the Account Id 4)Update the attachments with parent Id equal to the attachment Id (Query and iterate through each to change the parent Id and collect in list and update the List ) 5)Must be written in execute anonymous and executed (Try in sandbox first) 6)Hope you have less no of attachments not exceeding 10,000 ROWS

Map<Id,Id> mapofAcccIdIdbyContractId=new Map<Id,Id>();
Set<Id> setparentIds=new Set<Id> parentIds();
List<Attachment> lstattch=new List<Attachment>(); 
for(Attachment att:[SELECT Name, ID, ParentID WHERE Name LIKE '%.pdf']){
setparentIds.add(att.ParentID);

}
for(Contract ctr:[Select Id,AccountId from Contract where Id in:setparentIds]){

mapofAcccIdIdbyContractId.put(ctr.Id,Ctr.AccountId);

}
 for(Attachment att:[SELECT Name, ID, ParentID WHERE Name LIKE '%.pdf']){

if(mapofAcccIdIdbyContractId.get(ParentID)!=null){

att.ParentID=mapofAcccIdIdbyContractId.get(ParentID);
    lstattch.add(att);

 }


 }

update lstattch;

yes, history cannot be created since AccountHistory, CaseHistory and ContactHistory doest't support create() operation. See the following docs for more information. Account History SOAP API Contact History SOAP API Case History SOAP API

Your best bet will to create a custom object and store history information in it.