If you have Person Accounts already enabled and working and you have an existing Contact and an existing Account, can you somehow take these two and merge them, thus creating a Person Account?

Considering the following scenario:
Objects: Child__c, Parent1__c, Parent2__c
Relationships:
Child__c - Parent1__c (Lookup)
Child__c - Parent2__c (Master-detail)

I want to query for Parent1 and Parent2 (Many Parent2 will be related to Parent1) and I want to keep the relationship between them (introduced through Child__c). Child__c is just a junction object in this case.

I have 2 options:
Pseudo SOQL:

Select Child__c.Parent1__r, Child__c.Parent2__r from Child__c

or

Select Id, Name, ..., (Select Parent2__r.Id, Parent2__r.Name FROM Child__r) FROM Parent1__c

Because the relationship is not that simple, SOQL number 2 fits my purpose better.
However, I'm concerned of the performance of the nested SELECT statement.
So my main question is, are these cases optimized? This would be a regular JOIN in any SQL language.
Would I suffer any perofmance degradation in case 2?
Any reason to prefer option 1 over option 2 or vice versa?

P.S.
I tried using the Query Plan option on the developer console with the following results:
Nested SELECTs
Leading Operation Type: TableScan
Cost: 1.3
Notes: Not considering filter for optimization because unindexed. Table: Task__c Fields: ["IsDeleted"]
Adding a WHERE clause lowered the cost to 0???

No nested SELECTs:
Leading Operation Type: TableScan
Cost: 3.6
Notes: Not considering filter for optimization because unindexed. Table: Task__c Fields: ["IsDeleted"] 
 

If you have Person Accounts already enabled and working and you have an existing Contact and an existing Account, can you somehow take these two and merge them, thus creating a Person Account?