Can you really alter the derby.storage.pageSize property of the existing table? Or must i export my data, drop then re-produce the table using the new property, after which re-import my data?

The doc states:

Defines the page size, in bytes, for on-disk database pages for tables or indexes used throughout table or index creation. Set this property just before giving the CREATE TABLE or CREATE INDEX statement. This value is going to be employed for the duration of the recently produced conglomerates.

Therefore it are only used when designing the table and will also be valid before the table is erased.
You'll have to remove, create and re-import.

Bryan Pendleton pointed out create table as select but at the moment this doesn't work completely. Data can't be placed instantly.

CREATE TABLE ... AS ...

Using the alternate type of the CREATE TABLE statement, the column names and/or even the column data types could be per supplying a query. The posts within the query result are utilized like a model for creating the posts within the new table.

If no column names are specified for that new table, then all of the posts in caused by the query expression are utilized to create same-named posts within the new table, from the corresponding data type(s). If a number of column names are specified for that new table, then your same quantity of posts must trouble caused by the query expression the information kinds of individuals posts can be used for the corresponding posts from the new table.

The Without Any DATA clause identifies the data rows which derive from evaluating the query expression aren't used just the names and data kinds of the posts within the query result are utilized. The Without Any DATA clause should be specified inside a future release, Derby might be modified to permit the WITH DATA clause to become provided, which may indicate the outcomes of the query expression ought to be placed in to the recently-produced table. In the present release, however, just the Without Any DATA type of the statement is recognized.