Question: I have plenty of tables sharded by date. I am exploring the way to move on and use the new "Partitioned Tables" instead.
I have found this reference for converting tables: <https://cloud.google.com/bigquery/docs/creating-partitioned-tables#converting_dated_tables_into_a_partitioned_table>

However, as I have many processes relating to existing tables (loading data or Querying), I am looking for a phased transition approach.
Meaning: I would like to change the load process and the table definition at 1st, and afterwards, slowly change all relating processes.

1. How should I go about this transition? 
*(We have considered and ruled out the option to duplicate everything and maintain each process twice due to obvious reasons.)*
2. Is there a way to use the old table names convention (MyTable\_YYYYMMDD) when querying a partition of the new transitioned table?
3. Can I use the older Table wildcard functions to query it until I transform all my query templates?
4. Are there existing partitioned tables on a public dataset where we can experiment?

Thanks

Answer: 
This is not exactly quite what you want, but may serve the purpose. With standard SQL, BigQuery supports the equivalent of table wildcard functions:

Documentation for this feature is going to be available soon, but to summarize:

We provide a pseudo column \_TABLE\_SUFFIX that provides a way to address the table. For example, the following query allows you to read from tables myproject.mydataset.20150105, and myproject.mydataset.20150106 (assuming value1 is a field in both). 

SELECT \_TABLE\_SUFFIX AS table\_name, SUM(value1)
FROM `myproject.mydataset.*`
WHERE \_TABLE\_SUFFIX = '20150105' OR \_TABLE\_SUFFIX = '20150106'
GROUP BY 1;

For your use case:

You could first convert your queries against date sharded tables to use \_TABLE\_SUFFIX pseudo column. Then, when you are able to convert these tables to partitioned tables. After that point, you will need a minor change to your scripts to use the partitioning pseudo column (since the pseudo column name is a bit different in these two cases).
My few cents: 

> 
> Can I use the older Table wildcard functions to query it until I
>  transform all my query templates?
> 
> 
> 

Directly against new partitioned table - No. You will most likely get something like "`table wildcards matches no table`" 

> 
> Is there a way to use the old table names convention
>  (MyTable\_YYYYMMDD) when querying a partition of the new transitioned
>  table?
> 
> 
> 

The option I see is to create respective daily Views named after `new_partitioned` table but following `new_partitioned_YYYYMMDD` naming.  

So while having use of new partitioning feature you will still use old syntax before you do full migration.  

This comes with a little extra headache as you will need to make sure you have those daily views created daily going forward for new partitions - but if you are looking for temporary solution - this should be not big deal. 

> 
> Finally quick comment on reference for converting tables:
> 
> 
> 

It is not clearly stated in documentation, but looks like you cannot just use `bq partition` command to convert existing sharded to new partioned. First you actually need create that partitioned table. If not - you will get new table w/o schema so it will be useless. It might be a bug - dont know